[Ri2]

I wouldn't talk to them, like everyone above said, there is no point. It's happened to me and I've talked to the girl involved and she didn't know at the time, but once she knew, she was so into him that she didn't give a rats' ass about me.
Some people will feign sympathy like "OMG I HAD NO IDEA, ARE YOU OKAY??? WHAT AN ASSHOLE/BITCH/ETC!" then you find out later that they still continued contact with the cheater........ and ain't that a bitch.

The only real reason you want to do it is probably find some closure about the situation because maybe you feel like you didn't get that with your ex. You want to find a reasonable explanation for why this happened to you so you don't turn inward and somehow manage to blame it on yourself on a cold, dark lonely eve when you miss your ex.

Here's your closure, pal:

She (kinda) cheated on you because she doesn't care about you or your feelings. He let her do it (albeit may have been unknowingly) because he doesn't know you and therefore owes you nothing. Even if you did choose to speak to him, he doesn't have to explain himself to you. If she actually respected you, if she had an issue with your relationship, she would have communicated them to you before acting out. She makes bad choices in how to deal with her relationships. You don't need that in your life and are making a good choice in moving on.