I agree that you can send a brief "congrats" and see what happens.
And to follow off on ECB's last comment, YOU may have expectations as to what family does, but your BIL and this "friend" may have different expectations. Maybe they won't attend milestone anniversaries, etc. Maybe they won't (or won't be able to) help out his parents when they get old. And so on.
I think making any mention of putting it behind you or the fact that you'll be family now will make it a more loaded statement than anyone should be dealing with right now.
It takes TWO to have an argument. Be kind and respectful and never engage in an arguement. You do not owe them more than that. If you miss the friendship, then move forward. (However, if she has not had psychological help, things will repeat themselves)