Partners mom marked him as dependent on her tax return which is not accurate, is there any issues that can come from this?

Hello, I’m hoping to get some advice to relay to my partner. He moved out last year and got a really good paying job last summer, in another state. As soon as he moved out, his mom rented his room, to the point he hadn’t even finished packing before she rented it out. This is relevant because she has made decisions in the past to show she does care about money a lot, and has done things to hurt him in that way quite a bit. There’s many other stories with this, but I digress.

My boyfriend submitted his tax return and just got it rejected, and the reasoning was he was marked as a dependent. His mom had done it despite him falling under none of the terms. He isn’t under 19, he isn’t a student, he made more than the allotted amount, and he hasn’t lived with her for most of the year. Even when he did, he paid a lot of the bills and utilities because she didn’t want to.

What is the worst possible outcome from this and what should he do? He trusts his mom still, and marked himself as able to be a dependent on someone else’s return. Is this going to give her any portion of his refund? He doesn’t want her to get any of it, and seems to be positive she wouldn’t do anything to get money from his return. I’ve looked up answers on Reddit already and have only seen people saying “yes you can be a dependent” or not, I already know legally he shouldn’t have been marked as one. I need to know if she can get any monetary value that will hurt his return, as he’s worked very hard and doesn’t want that to happen.