That cheating husband is self-destructive on many levels. It's bad enough to cheat on his wife, but to try to seduce his wife's best friend (or close friend)? If he had HIS way, he would create a terrible situation (if she were to suspect or find out about his cheating) and chisel away at her support network (her friend).
He really is abusing YOUR friendship--he can logically predict that you will keep quiet and stop being friends with her out of guilt, or tell her (maybe he is so guiltridden he wants things to explode in his face, for her to divorce him and leave him free to openly pursue other people?) What a dirtbag!
I can understand your being afraid of the repurcusions of telling the wife, since he's a cop. He's already shown that he isn't a decent person.
Maybe he hops from lover to lover because HE'S bad in bed and THEY dump him?
I just bet that his extreme risk taking behavior will worsen and he will be exposed somehow, even if you do nothing. I feel bad that he put you in this predicament in the first place.