He doesn't need to show proof that a 3rd party could have presented it to her because He WROTE the song and would know who HE WROTE it for. If she said it was presented to her and she turned it down then maybe she'd have a leg to stand on. Something tells me though at this point in her development she doesn't have say in what she will or won't sing. Maybe after her first album is a hit then she has the clout to make such choices. It comes off as rude to have an artist who hasn't done much to prove her own chops try to say she refused his song when that probably wasn't the case.