WHY WONT ANYONE ANSWER?!?

It really depends what kind of guy he is. If he is a good guy who really cares for you and is just being nice to his best friend by saying she is pretty then its not so bad, but if he has feelings for her then its different. Either way I think since he is your boyfriend he should have some respect for your relationship and not go around calling other girls pretty, I don't think its right and I am a guy. If it is bothering you let him know.