Jack did tell me he and Diane talked about marriage before he proposed, so maybe they have discussed some of those important issues. I guess most of what’s worrying me is that I’m under the impression that he hasn’t really thought this through and just kind of made this life-changing, spur-of-the-moment decision. That probably isn’t true, though. I think when I see him in a couple of weeks I’ll take him out and ask him about it. Not in a bad way, either, but in more of a supportive, “When did you know she’s the one?” way.
I don’t plan on being anything but supportive for him. The last thing he needs is to be second-guessed by his closest friends while his marriage approaches.
I think the real cause for concern which many people seem to be overlooking while telling heartwarming stories is that he’s done this before. Still, sounds like you have a good plan for what to do, so best of luck to you and them.
I completely forgot that I named them Jack and Diane, so when I saw your post, I thought, "John M.? 
" Funny, though, when I clicked the link, an ad popped up. It was the one for classmates.com or whatever that has old yearbook pictures of a guy and a girl and says, “SHE MARRIED HIM?!?”
Six months here. We were so young.
But, almost 12 years later, we’re still young, no kids, and man, things just get better every day. I’d say the first five years were the hardest, but you’ve got to be committed to making it work.
I also second the thing about the disapproval of friends. See, my husband was married before me. I knew of him then, but didn’t know him well. His first wife died at 22 from complications from chicken pox of all things. We shared all the same friends, and when these friends found out that we were getting married six months after her death, they went ballistic. They weren’t angry so much with him as me. I was really persona non grata for a very long time. But friendship, like marriage, if you remain in it for the long haul, can be one of the most rewarding experiences in your life. The two most vocal people against our marriage 12 years ago are now our best friends, and I wouldn’t trade 'em.
We got lucky, but then again, We’re always above average when given a 50/50 shot! 
I wish the best of luck and fortitude to these two if they go through with it. I love a good underdog story.
I understand your concern, but really, the wedding is over a year away! That’s a lot of time in which they can figure out if it’s what they really want. If he shows signs of unhappiness in the last 4 months, start worrying then. With his track record, I doubt that he’ll be the kind of guy to stick out the wedding day if he doesn’t actually want to.
(We got engaged after four months, which I considered to be terrifyingly fast, but DangerDad had made up his mind after 3 weeks that I was the one for him, so he was just waiting for me to agree. We married 8 months later, and had our 10th anniversary this year. We are disgustingly happy. I think a lot of this marriage stuff can almost be the luck of the draw; lots of people get married fast and are happy, and lots of other are prudent and slow and miserable anyway. Though I would still advise at least a year of dating before marriage, I’m not sure it matters as much as we think.)