Okay buddy, here's what you do.
About your friend chatting to your bird:
IGNORE HIM! Let him do what the hell he wants. You're the alpha, you're in charge, he's just an AFC and he's making himself look bad by talking behind your back.
Your GF loves you! So it doesn't matter what anyone else says to her about you. She's not gunna be turned off by shit-stirrers. She's in a relationship with you so she is on your side.
Try not to let it get to you!! The second this guy notices that it pisses you off, the more he's going to do it. Don't confront him just yet, if he knows he's getting a reaction, he'll persist. Be happy!! Because sooner or later he's gunna give up.
About guys 'hitting on' your bird:
Treat this as a shit test. Often girls blow it out of proportion a little but even if a guy was hitting on her it doesn't matter. Just remain alpha and ignore it.