What can I do when my my daughter's boyfriend is taking advantage of my mother-in-law?

I don't think so

She needs to be seen by a doctor. Do not tell her it is to have her evaluated, I wouldn't want to go either. Imagine how frightened she must be. Send a note to the doc before the appointment about what has been going on. Then doc should take the lead in the examination. Is she seeing a geriatrician?

she definitely has dementia but has not been diagnosed by a physician.She has been saying inappropriate things to my daughter's boyfriend.I have noticed her hair looks dirty and she isn't combing her hair and last week she backed over a light post in her yard she denies it.She insist she will let us know when she can no longer drive.I have been bringing her to appointments that are a town over because she said it's hard to park. She has a fear of falling she won't go to a doctor to be evaluated.

If MIL is mentally competent, and the $200 per week doesn't put her in any financial danger, does it really matter if he does 2 hours or 20 hours? He is helping her and she clearly feels helped, unless she is not mentally sound.

I would not hesitate to ask your daughter about it. Not in a confrontational way. Ask what he does to earn the money and if she thinks it is fair and reasonable. Is MIL competent? How old is she? If she is competent she can spend her money any way she pleases.