Support him with his girlfriend, that way you win both ways 1. If he stays w/ this girl, then he's happy and if you really love him thats what you want, and you still get to be his best friend
2. If he breaks-up w/ her, then your right there, and have always been on his side, and then you can tell him how you've really felt the whole time
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"I reject your reality, and substitute my own" -Adam Savage of Myth Busters