I was raised to mind my own business but when someone asked you an opinion to be straight...okay, here's my opinion...WTF do you care who he sees and who she sees? He didn't make a play for her while you were with her and certainly didn't try to pull any fast ones behind your back...he seems like a pretty good friend. One could reasonably argue you dated this chick to teach your ex/then not ex a lesson...so if she is seeing your friend to teach you a lesson...feel bad for your friend. On the other hand, maybe, just maybe, your friend and this chick really like each other and they are very happy together...why not be happy for them? Or were you planning on scooting back to her if the ex/not ex got out of line?
Life's too short...no?
Now as for the lies he told about you...that's ass kicking grounds