One of my friends dated a guy for 7+ years that I HATED! Hate is a strong word but he really didn't treat my friend well at all. Plus all the time she used to spend with me, she started spending with him.
I am happy to say she has moved on and is dating a new, awesome and amazing guy (who I love)! But it's not really up to us to pick who our friends end up with. In my opinion, as a good friend you go along with whatever makes them happy as long as it doesn't put them at the risk of being harmed. So I never told my friend that I didn't like her ex. I thought she was happy and that was enough to make me happy. When they broke up I told her how I felt and she said she had been unhappy for awhile.
Matt Damon has also waited a really, really long time to say this about his BFF Ben Affleck's time with ex, ex girlfriend Jennifer Lopez:
I don't regret not telling my friend what I thought about her ex-boyfriend only because I'm sure she did have some happy times and it was also a lesson that probably led her to this new cool, loveable boyfriend. But if I could I would go back and ask her if she felt valued in her relationship. Maybe she wouldn't of shared her true feelings right away, but I would ask her again. And again. Just so she knew that she could tell me ANYTHING that she needed to.
That would be my advice to you. And Matt Damon.