You can’t go wrong with a dark suit. Unless it’s a beach wedding, I guess, but I feel you would have alerted us if that were the case.
If you don’t have a dark suit, you really should get one if only for weddings, funerals and job interviews.
The same outfit as the Bride, silly!
Surprised to see so many people saying suit. The few weddings I’ve been to, only the wedding party dressed up in a suit. If you’re just going to be among the throng of people in the back rows, you’re fine with a button-down, long-sleeved collared shirt, some slacks, and dress shoes. Tie optional. If you’re closer to the participants, and sitting closer to the front, maybe add a sports coat.
EDIT: Of course, as others have said, best advice is to ask the girlfriend or bride. It’s their wedding, if they want you to come dressed up like a Star Wars character, that’s what you should be doing.
In some outdoor weddings, light colored slacks, white or pale blue shirt, and a navy blazer.
Slacks, a dress shirt, and bring a tie and a sport coat will always work. You can always take the tie and coat off.
Nice plaid shirt with pearl snaps, bolo tie, and a Stetson hat. Brush the manure off your boots.
See, this is why I held back from answering this thread as I’m starting to realise that there’s a gulf between what would be expected at a British wedding vs a US wedding. In the UK, a lounge suit and tie would be mandatory unless there was some crazy fancy dress theme or so posh that all the men were expected to dress like this (usually reserved for the immediate wedding party).
I would simply ask if there’s any particular dress code, or what they expect most people to be wearing. The weddings I’m used to have normally been either “mostly suits, some smart casual, wedding party in morning suits” or “dress up fancy but it doesn’t matter in what” depending how traditional they are, but I don’t expect that’d be representative for you.
It’s really mandatory in the US too for a wedding of any formality, there’s just a lot of Americans who… every word I think to use to describe the phenomenon would get me flamed by the “Daisy Dukes to a funeral” people.
For a spring/summer wedding, wear a light gray or khaki suit, unless you’re twelve. The tie is probably optional.
Notice I said khaki suit. Like this. If it doesn’t need to be dry cleaned, it’s not a suit.
I’ve been to multiple weddings in the US and the UK and I have only been to one where ties weren’t expected for men. And that was on the beach in Hawaii.
If it is a purely indoor affair, I’d do a suit. If the reception is outdoor, and the weather is going to be nice, a blazer with grey pants is perfectly acceptable.
You’re crazy. As I said, I’ve been to a dozen wedding and nearly all of them were of the Italian variety. Suits were only required for the bridal party and various close family members like the parents. No one else batted an eye when others just attended in dress shirt, dress pants and dress shoes.
Even U.S. weddings will vary in formality depending on what part of the country you are in, time of day, venue, and theme. Around here, it isn’t uncommon for the suit coat or sport coat to be worn to the ceremony, and ditched for the reception - tie quickly to follow depending on how quickly the bar opens.
But a dark suit barring something that says something like “backyard luau and pig roast” is usually a good bet.
Part of the issue here is that the OP apparently doesn’t know the bride very well (“roped in by his girlfriend”) - and in the case of meeting strangers, it’s better to err on the side of conservatism. We had two guys attend our wedding in jeans and T-shirts - one of them had had either the airline lose his suitcase or the overnight dry cleaner turn out not to be so overnight, I forget which, while the other guy was someone my wife had known since high school or maybe earlier and who NEVER wears anything but jeans and T-shirts to anything. Ever. So that was all cool.
“Daisy Dukes to a funeral” people?
Frankly, I’ve been to weddings where the groom wore neatly pressed jeans and a starched white shirt (small, home wedding) to white tie affairs. At most of these, a dark suit wouldn’t have looked out of place, but wasn’t necessarily required.
I’m still sticking with letting his GF choose or at least asking her opinion. 
But you remember those guys, and what they were wearing. They stood out, and still do in your memory. Surely the point is that the OP, not knowing anyone really apart from his girlfriend, just wants to blend into the crowd and not embarrass her.
Well not crazy but certainly different experience than you’ve had. There’s a big difference. Where are you from? I’m from the mid-Atlantic area and the vast majority of my family members are blue collar union-folk. I’ve been to a bunch of weddings with them and none of them would even consider not wearing a suit. (we don’t get invited to a lot of beach weddings so I don`t know what would happen in that case)
In fact, when I’ve seen anybody at a wedding without a jacket they’ve looked really out of place if only because they were one of the only people not wearing a suit.
I’m willing to believe there are places or groups of people who don’t dress up for weddings but I’m quite certain Zsofia and I and everyone else who has experienced differently aren’t crazy. And again, I’ve been to weddings with fancy-pants people you’d never expect to not be in a suit, but the majority have been largely family affairs and we’re very much a working class family, still in suits.
Western New York.
Maybe we’re different here in the South, but a respectable man would wear a suit at least to any indoor wedding here unless the reception was held at Dirty Pete’s Billiards n’ Porn.
I told my wife that wasn’t a crazy suggestion, but noooo, we had to have it in the Parish Hall.
I might be alone in this, but I hate suits like that - there’s nothing like a light brown suit to make you look like an accountant (especially with the modern suit cuts that are cut to look too small, so you can look like an accountant who’s suit is too small). I’d much prefer to see my husband wearing dark dress pants and a dress shirt than a brown suit.