I don't think it matters whether or not you like him.
What your husband did to him was unthinkable and I would not want him there.
Best friend? No. Friends don't behave like that. (SAs don't really have friends when they're steeped in their addictions, anyway. They have tools---people they use to feed their addictions.) It may be "past the point" of telling the OBS the truth, but your husband helped ruin his 1st marriage. He has no place at the wedding. And because you knew and did not tell, I'd say that you should stay away, too.
I know your intentions were not bad. You were coping with YOUR end of a bad situation, and doing the best you can.
But the OBS, were he to find out, would likely not view it similarly. If there's any chance he might feel as though his new wedding day was polluted, if he were to learn of the betrayal, it's reason enough to send regrets.
I'm really sorry