I wouldn't recommend going any further with your boyfriend's friend. Doing so would really screw things up.
I've seen similar situations where it all goes to hell simply because this happened.
Let's look at it this way... Jack really loves Jill as his girlfriend. Jack is best friends with Jake. Jack enjoys being with Jill, but also enjoys doing guy things with Jake. Jack and Jake have a lot of fun together and they trust each other.
Suddenly, Jill decides to go after Jake. Things are already progressing between Jill and Jake before Jill and Jack even break up. When Jill and Jack finally break up, Jack is devastated by the breakup. Jack eventually finds out that Jill is seeing Jake... and it was before they broke up. Now, that hurts Jack even more... his own best friend doing this to him.
I have a lot of close friends who date women that are the type that I would date. But I keep my distance and ensure that it remains in a manner that we're simply "friends". If they try to hit on me, I keep my distance so that I don't hurt my friend... because my friend is more important than his girlfriend.
If my friend broke up with his girlfriend, then I would have no problem going after his now ex-girlfriend.