Your boyfriend reacted well. You could tell him you saw it and that you're proud/thankful of how he reacted. I'd definitely talk to the friend though. She did a big no-no.
The best friend is now a non best friend and cut all ties with her
B!*(# slap that B!*(#
Sounds like the makings of an awesome *********!
Personally I would slap that ***** and start a big screaming match with her but that's probably not a good idea.
But all I can suggest really is just to cut off all ties with her because she's possibly the worst friend ever. You have a good boyfriend though, keep him.
Say you saw what happened, and make sure he knows you're happy he reacted that way. As for the best friend... you may want to rethink her best friend status. Ask her why she did that, and tell her how it hurt you.
I wouldn't say anything They both showed you, one good boyfriend, and one bad friend!
Just confront her for now, then decide what to do.
confront her and explain what she did is wrong, she's broken your trust, and even hiding it by not telling you so not a very good "best friend"