Having sex with a guy whose taken

Hey, I feel both guilty and turned on by this but I'm fuck buddies with one of my best friends who has a gf and I can see his relationship failing cause she avoids communicating and she doesn't even touch him or do anything with him for months and I feel bad but sex with him is amazing and I kinda want to date him but it's like he doesn't want to leave her cause she's apparently like his best friend and he loves her and they have this whole plan of moving in together getting married then travelling the world and after 6 years having kids but I don't see her going through with that at all and I think she may be cheating on him cause how else can u explain her not wanting to have sex with him after 2 years of being together when he's so good in bed aswell