My one cent on this topic:
If you receive an invitation, I think it would be unwise to go - you're opening yourself up to a lot of emotional garbage, none of it positive. Would the best case scenario be that you sit there, mildly uncomfortable while she gets married to the guy she cheated on you with? That sounds like it would be painful and unnecessary. Don't go to the wedding, but do acknowledge the invitation. I would send a card (no gift) congratulating the couple and wishing them luck. It's relatively impersonal, won't piss anyone off and only requires a small monetary and emotional investment on your part. The happy couple can't say you didn't make some effort, and if this girl has a brain in her head, she won't push a situation with the potential for emotional pain on you.
If, by some weird alternate-universe reality you do decide to go, you will be miserable - having been there, done that, I can pretty much offer you an iron-clad guarantee on that one. I hate to say this about my own gender, but some women (okay, really girls) have this idea that they can round up all their exes and stand there french-kissing the guy she dumped them for, then expect all the ex bf's to still fall all over themselves wanting to be her friends. Sometimes it just doesn't work that way, and in the rare instances when it does and friendship is possible, it takes years to get over the bitterness and hurt feelings from the failed relationship.
I'm not saying you can't be friends with an ex, but it's a different situation when that ex has cheated on you and expects you to offer her happy thoughts while she marries the schmuck she cheated with.
Lastly - should you tell her what you think of the schmuck? NO! She doesn't care what you think, unless you plan on offering your seal of approval. She may ask for the truth, but what she wants is you to tell her she's doing the right thing. At this point, it's her mistake to make, and there isn't anything you can say to her to make her dump him without her blaming you.
Hopefully that makes sense, Vicodin has the weirdest effect on me. Good luck, hpkeeper!