Friend (or supposedly friend"s") left me because of her. What to do...?

Ok so it started when i introduced my girlfriend to my group of friends about 3 months ago. She was very nice and cheerful towards them and they seemed to like her pretty well! (except for one who liked her a bit too much) But anyways, things started to change, her brother recently went to boot camp and now she started to be a bit depressed...well one of my friends noticed and started to not like her, so he started ignoring me, and began to be rude to her. Well since hes the "alpha" in our group, another one of my friends began to leave me for him, and so on. Ive almost lost all my friends because of him! What do you think i need to do?