OP is not in the band.
Different strokes for different folks.
My ex told me that since she was wasn’t able/willing to keep up with me, it would be okay for me to have sex with someone else as long as I didn’t fall in love with the other girl. Never did, but it was on the table.
A friend of mine was having an affair with a younger guy. She told her husband about and she said when she got home he’d ask for details about what they did and got turned on by it and they’d have sex after. I cut off contact with her because I knew her husband casually and told her she had to choose one of them without my advice. A couple years later, she contacted me and told me she got divorced and married the younger guy.
Jim: “Sue, I’m in a hotel room with a 22 year old and we’re going to bump uglies.”
Sue: “Okay, have fun!”
Another friend was having an affair and his wife (whom I had a brother/sister relationship with, I knew him first) would call me over to console her because he told her. My friend was totally aware I was going over to his place when he wasn’t home (I’d been alone with his wife before while waiting to meet him to go out). I tried to play counselor (and told to get professional help, he refused) and things got a little better between them for a while, but he went back to his lover. The last time I talked to them together (he came home when I was at his place), I asked him to make a choice. He said he wanted both. I left after saying that if that was the case, they might as well get divorced. They did a few months later after his lover revealed she was just using him because he was helping take care of her son.
Different strokes for different folks!
Edit: He claimed it was her fault because they stopped having sex (they were in their 50’s) and she never said expressed her love for him. During their reconciliation they started having sex again and she’d tell him she loved him, but he said it wasn’t real. I told him she did it because she really loved him and wanted to save the marriage.
Look that one up under “Ono, Yoko”.
To all the people saying stay out of it, I’m glad you’re not my friends. If people just let their friends risk ruining their lives without helping, because “it ain’t my business”, then what kind of friend are they, really?
If I had a friend that was cheating, and I confronted him, and he got mad and dumped me as a friend, well, so be it.
Sorry I meant his band friends - was aware of that but thought, wrote hastily.
Heh. He’d laugh in my face. I’ve had a fling with a much younger woman (before I met my gf) and I’ve related the story about how embarrassed I was after discovering her age. Also <humble brag>my gf is way outa my league</humble brag> and that is a source of good natured joking between us.
I can’t out the guys who told me about the situation, I promised them that before they told me.
**Consider yourself fortunate. **
Hormone fueled relationship conflicts can be very intense. It isn’t wise to place yourself in the midst of such a conflict. It’s a good way to get yourself assaulted or even shot.
I’ve had two different guys who are policemen tell me that the calls they hate the most are domestic disputes because they are extremely unpredictable.
OP doesn’t KNOW anything. It’s just rumor and hearsay at this point. It’s not OP’s job, or anyone’s, to referee other people’s relationships.
If I were the OP and Sue asked me directly, I wouldn’t lie, I’d tell her what I know, but I’d never just volunteer it. Right now this is all pure gossip.
What if Jim’s not doing anything at all but you horn in and go tell Sue that he’s cheating on her, based on just this gossip? Is that being a friend, or is that instigating shit that’s none of your business?
Yep, I’ll probably get shot by a jealous partner someday because I’m Mr. Listening Post. But at least I’m not Mr. Listening Post and Tell. I always end the conversation with, “Okay, now you tell him/her.”
BTW, for those who posted on my other thread where I said if I’m in a relationship, I need 100% disclosure and honesty, yes, I would tell my partner if they asked. Never been in that situation though in the past 25 years.
Count me in with the “don’t get involved” crowd. This is a Jim and Sue problem, not a kayaker problem.
He knows he is wrong, and if you confront him, finger-wag at him, and tell him that Jesus does not approve, it won’t change anything. He knows it. He’s a big boy and he will have to figure this out for himself and deal with any consequences.
As far as Sue goes, you are not the marriage police; don’t volunteer for that job.
Relationships break up all the time. Relationships run into roadblocks all the time.
This is not a tragedy. It’s just life.
And for Pete’s sake, these are traveling musicians. They should keep their noses out of their bandmates’ intimate relationships.
Sounds like a mantra I need to concentrate on.
His fellow band members should stop pussyfooting around and just ask the guy “Wassup, dude?” (or if it’s a country band, select different verbiage).
For a subtle approach, suggest “Jim” perform this solo at their next concert.
Or maybe it’s time to rip the scab off the mess entirely. A few anonymous phone calls to relevant parties (use a burner phone) will get everything out in the open and allow the healing process to begin.
Why do I think this is a “Why does he get the hot 22 y/o and we get nothing?” rant by the bandmates?
OK, if playing bluegrass isn’t enough, have him switch to banjo! 
The only thing I might do was go directly to Jim, assure him it won’t go any further but maybe attempt to have a heart to heart talk with him. I ran a trucking company for several years and it wasn’t unusual for truckers to get involved in flings and then think they were in love. I had many heart to heart talks with truckers over the years. Most of the time they just keep doing what they are doing but sometimes a wake up call helped them to make the right decision.
I would advise the band members that this is not my problem. And that if they really want to make this their problem, first be very sure they’ve got their facts right, then take it up directly to Jim and nobody else.
I wouldn’t want to see a friend doing something like this. But I wouldn’t feel like getting involved in what’s probably nothing more than a road fling. Not that road flings are OK with me, but unless he’s risking life-changing consequences, I’m not getting in the middle.
I would advise the band members that given this information, all their drinks are free until such time as Jim-bo knocks it off with the sad & pathetic mid-life crisis bullshit.
Not even close. They (like me) have sown their wild oats and we know how this will likely end up. 
Jim knows how this’ll end up, too. He just doesn’t care - thinking with his little head - and you can’t reason someone out of a position they didn’t reason themselves into.
There’s nothing you can say to convince him to knock it off. What, he forgot he’s married? Amnesia at being the kid’s father?