Firstly, put it this way, if I'd bought it I'd be fuming but then at the same time my eyes would be my judgement before I parted with any cash. Its up to you what you do but personally I'd give him his money back as it doesnt declare any sold as seen on your advert or like Chip says "spares or repairs" which is what it should be if it is correct that the frame is bent.

Secondly I'd also be asking my mate why he sold me a bike with a twisted frame