I know how you feel, both in feeling alone and in being miffed about your friend buddying up to someone that's hurt you like that. I too find it petty to tell friends that b/c a person is not cool with me, they shouldn't be cool with them either --- even if that is how I feel. It seems disloyal to me when a friend does that. I know exactly how you feel.

Sorry, no real advice on this one. But I can relate. I hope you're able to find new, better friends soon.