My mom is 70 and is putting weight loss over staying happy

She's gone crazy with this keto diet stuff, she's going to the gym every goddamn day, and she is seriously taking money from her retirement account to pay for it. She's obsessed and the worst thing is she isn't even a fat person by any standard. We had a conversation where she said that all she thinks about at night is all the weight she has to lose. She's also spending ridiculous amounts of money on gym classes and all kinds of diet food, so much so that she had to put it on her credit card. I'm *so* worried about her but I don't know what to do. Any advice?