[NSFW] Is it morally wrong..

I'm a guy, into guys. There is a guy I know fairly well who has a girlfriend, but wants to hook up with me and have me dress up in his girlfriends panties etc, etc. I think it sounds hot and I want to do it, but I don't know if I'm comfortable going behind this girls back like this... What do you guys think? Should i feel bad about it if I do it?