It's a difficult situation, Alex -- and I always hate to give hard answers.
I think the "standard" for what a Catholic should attend is: "Is this marriage even valid?"
I think it's one of those things where you should go out of your way to be charitable to him (and his "wife"), but gracefully not attend the wedding. I think you know what I mean. There need be no fireworks; you can be calm and cheerful the whole time.
If you don't attend the wedding, yet you don't yell, argue, etc. but on the other hand are very charitable to them -- it will be the most you can do to make them stop and think. By acting like a saint in this way, they won't be able to easily pigeonhole you.
My wife and I already agreed that we won't be attending any "second marriages" of any friends or family. That's our cutoff. We've been to several Lutheran weddings (college friends of my wife, and one sibling of hers), but they were a first wedding for the parties involved.
I mean, those people are non-Catholic (or fallen-away Catholic), so they objectively commit a mortal sin every Sunday by not attending Mass -- but do they commit a mortal sin subjectively? Do they realize their obligation before God, and that only the Catholic Church can save them from Hell? Only God knows.
I think a Catholic's goal should be "How can I make the maximum good impression of what Catholicism is all about." and, of course, it goes without saying that EVERYONE should think of Catholicism when they think of you. If they don't, you have a bigger problem.
We have relatives that know instinctively that inviting my family to dinner on Friday means they need to have seafood or vegetarian fare on the menu. And I bet if anyone asked many of our relatives about us and birth control, I'm sure most of them could answer confidently that, "Oh, they're Catholic...I mean really Catholic...they don't touch the stuff."
I think that's a very good thing. It can be uncomfortable to be different, but it's worth it when you know you're doing your job to witness to the truth.
Matthew