That's ... a problem. And there's no easy fix I know of, in post.
Now, if say there was a rimlight from each side of the set during the shoot, it wouldn't be an issue. Just a single smallish light near the edges of the background pointed at the subject, maybe a stop or two less light than falling on the face. You's have a clean edge no matter what the subject wore.
As is ... I'm not thinking the shirt would look correct if you lightened it, and the background wouldn't work if it was darkened. This is a beast to solve now, and I've got little help to offer.
It's not going to be easy to get them to do that. I was just hoping there wouyld be a way to take the dark green background and differentiate it a little from the black shirt he is wearing (Premiere or AE), but if there isn't a way to do that, I may be out of luck.