Is my best friend's boyfriend being friendly with me, or inappropriate?

So my friend cheated on her boyfriend with his best friend. Since this has happened, they've all talked it out. The boyfriend's buddy is out of the picture, and the couple stayed together. But lately I've been noticing that the boyfriend has been very flirtatious. With me. Since he and my friend have been dating, we have been pretty good friends too. I began to think of him as a brother figure, but since the whole cheating incident, he's been very sexual in his play and jokes. We used to roughhouse, now he throws me on the bed and pins me there by the wrists. If I'm sitting on my bed he comes to cuddle, or rest his head in my lap. The other day when we were standing in line for coffee, he held me and kissed me on the forehead. He walked me to one of my appointments but told me to tell my girlfriend that he went to class.The lie made the whole thing feel too intimate. He also talks about what a great ass I have when she's sitting right there!

My other friends notice and they all think that he's trying to use me to make her jealous after what she did to him. I never reciprocate, but I think she's starting to get annoyed. Recently he forgot his phone at my apartment. She found it and started rifling through, - as most insecure girlfriends tend to do - and began to look confused. She laughed it off but said; "There's a picture of you in his phone." There was a picture of me there, but it was obvious I didn't know it was being taken. I can't even pinpoint WHERE he took the picture or how he got so close without me seeing. I felt creeped out, embarrassed, and guilty like I'd actually done something wrong. I feel like I can't talk to him about it because it would make things awkward. Especially if he was just being friendly this whole time. Is it nothing? Should I just brush it off? Or is there something? And if he is flirting, why is he using me to make her jealous?! Does that mean he never really liked me as a friend? Help. I feel like a pawn.
Is my best friend's boyfriend being friendly with me, or inappropriate?
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