Should I tell my boyfriend???

So I've been with my boyfriend for a year and before that he was in another relationship with a girl that has a lot of trust issues and she was constantly flirting with other guys. In fact, one of those guys was my boyfriend's best friend at the time. They stopped being friends because very obvious flirting between the two at a party. He dated the girl for a while longer and they eventually broke up because the relationship was wildly unhealthy. Shortly after that we began dating a mutual friend of mine and the ex best friend told me that he's been sleeping with my boyfriend's ex. At the time I didn't tell him because a) it was none of my business and b) I figured it would cause unnecessary pain.

***Skip ahead a year***

My boyfriend is thinking about forgiving the friend but he doesn't know that he was right about him flirting with his girlfriend at the time or that he slept with the ex right after they broke up. Should I tell him now?