Yaz, who is 21 [and a stripper], is dating Greg's friend, who is 42.

Come the hell on.

One time his roommate even asked if he was just jealous of the fact that Yaz chose his friend over him.

Yeah, sounds like it!

I also get the feeling that Greg is her backup-BF, as she often goes straight to Greg when she's fighting with her BF, complains about her BF to Greg, and would rather draw with Greg than with her BF, who is also a fine artist.

Greg and his friend are splitting the job of BF. Friendo is the "daddy AF" bedroom needs guy, Greg is the emotional support animal.

I was a nude model for ten years. I am an artist, lived with artists and worked in the fine arts. This isn't normal. This situation sounds gross. There is a serious case of poor boundaries. Yaz should not be discussing relationship stuff with your BF-- your BF should be pushing her off by saying "hey, he's my friend, don't tell me this stuff." Moreover, they should attempt to retain a fragment of professionalism if she is modeling for him in the studio with other students . . . She probably shouldn't be modeling nude for her boyfriend's best friend who is also her emotional support crutch slash teacher, either, there should be a boundary there.

Ask your BF if he can go two days without talking about Yaz. If he can't, well, I'd say there's something wrong there.