The problem is as others have pointed out, sometimes people have the kind of relationship where if you take it out of context, it sounds abusive. If you have ever been around certain groups of male friends, you will see that kind of thing, where putting down who they have dated, their mother, you name it is all fair game, and it means absolutely nothing.
In this context it is hard to tell. I would be concerned as the parent, because sometimes what others think is being said in jest isn’t, and sometimes you can’t rely on the opinion of the other person. I am sensitive to stuff like that, because I grew up in a family where sarcastic comments and put downs were supposed to be ‘funny’, but they in fact were cutting and hurt, well into adulthood, it wasn’t until I got into therapy then had my own child that I realized how cruel it was.
It could be nothing, but some things stood out for me with the OP’s post. The fact that the GF has body issues (I am assuming here that she has expressed issues with her body, rather than someone assuming she does because she is somewhat heavy) would be telling to me, because people with self issues will often lash out at others, to ‘bring them down’ to their level, or as in the original post, for example, belittle the other person for something they aren’t as good at, self conscious of, like for example intelligence. When she made fun of him for changing his shirt, that to me could be her being jealous if the S is (in her eyes) more ‘better looking’ in terms of his body, and trying to make that into something to be laughed at.
To be honest, I can be very sarastic with friends, but someone I was in a relationship with I would never tolerate that kind of thing and I certainly wouldn’t use it. There is a difference between playful teasing or inner joke sarcasm (for example, if my wife talked about being an animal trainer, and I made the sarcastic remark “yeah, you would be another horse whisperer”), she would laugh, because she knows she is a smoosh pot with animals, our pets are spoiled rotten:) and cutting remarks, and the case I heard was cutting.
I think as a parent I wouldn’t bad mouth the girl, but I would talk to him about defending himself and about how in a relationship it is not okay to hurt the other person deliberately or treat them with disrespect, and that sometimes it can be hard to tell. The S probably really likes the girl, probably feels like he was so lucky to have such a girl like him, but there is the concept of boundaries and being treated the way you want, and knowing the difference. We have been going through this with our S (who is in college, not hs), with a relationship where the girl’s behavior was inappropriate (in his case, lack of self confidence, constantly texting him, wanting to know where he was, getting mad if he did things when apart from her that she would have wanted to do but couldn’t), and we talked to him about the behavior and how it wasn’t good. We were careful not to make it about the girl being bad, or not liking her, but rather pointing out how that behavior was hurting him and their relationship. If you say “that girl is no good for you”, they’ll get their back up, if you rather say “I am concerned about her behavior” and talk about it, it isn’t a direct attack.
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We intend to be as neutral (or positive) as appropriate down the road when our kids start having serious relationships with S/Os. So far, we haven’t met any, so will see how things evolve.