He knows how I feel and he's seeing my friend

I've liked my best friend for months. He knows how I feel but when my friend asked him out (she also knows) he said yes. Me and her decided to check with each other before either of us asked him out so the other one wouldn't get upset but his girlfriend dumped him this morning and a few hours later she asked him out. We are only teenagers so everyone will say get over it because it's just a little crush...as if they never had problems with teen love. How should I react? Also she split up him and his ex.