Please only knowledgeable people respond, thank you..I have a question on taxes?

I am asking for someone else, as this person has been separated from his wife for two years now. He makes more than her and is wondering if she has any claims over his earnings? She wants to file jointly and he does not want to. I am thinking that since they are both separated from each other, he should be separate from her and likewise. Kind of sounds to me like she wants more of the cake then she is deserving of. They have one child that is now an adult so child support is not in the ball game any more. Anyone else have some opinions on this, would appreciate it. Thank you in advance.