Irriated

My mother in law likes to ask my fiance to help her around the house so she can save money to go on trips and online shopping. I understand helping your parents out but when you have them do really difficult manual labor that he doesn't know how to do and put him in danger and takes time away from his 1 year old son and his fiance, plus the job that he's about to go to is a problem. She wants him to follow orders of a 70 year old man who can't work on her roof because he's too old but has experience to put something on the 2 story house to salvage it for the weather and then when the weather is better, her home owners insurance will cover it. She doesn't want to get anyone to do it because its expensive and because if someone falls off, they can sue her. What about her son falling off ? The time, the fact that he's doesn't know? The pay? Last time she told him it would take 4 hours and her other project took 3 days. Hes blinded and says he's going to have to because she can manipulate him. I don't know what to do or say. I've told him I concerned about his safety and the fact that he just started a new job and has us and he doesn't have medical insurance. No matter what I say or do. Shes not going to care and he's going to do it. It just makes me sad .