Best friend kind of cheating
My best friend (guy) has been on and off about four times with my other best friend (girl). Well, in the downtime between their breakups, my guy best friend texts my girl best friend's best friend dirty things, asking her to do sexual things with him, and is overall being inappropriate. He doesn't know I know. His on/off girlfriend doesn't know either because her best friend won't tell her, even though she doesn't act on anything he says and continues to tell him no.
Anyway, is this normal for him to be doing this? What should I do? I don't want to ruin any of my friendships, but I also don't want to just sit around and watch this happen because I do care about my girl best friend more, and she can never seem to get over him and goes on and on about how she trusts him and how much she loves him. I don't want to hurt her, but I don't want her to be wasting her time with a jerk... even if he is a close friend of mine.