My Boyfriend's best friend called me beautiful - in front of my boyfriend?

You need not do anything. It'll remind your new b/f on how lucky he is to have you by his side. Other than that, take it as an ego boost and let the new b/f watch his step as far as how he treats you, for there are other sharks in the sea waiting to take a nibble if he doesn't treat you well. On the flip side, if you do bring it up, it may just end up causing you, him, and the best friend problems that you didn't ask for-- so don't.