This might sound weird but I don't think you should jump to giving advice. There's obviously a lot of history that you weren't a part of so really she needs to decide for herself how to handle it. Plus your attraction to that body type could mean you are biased.

Possibly ask questions instead. Maybe ask what she'd like to do, ask if there's anything you can do for her, stuff like that.

Also if mother is European and you two live in America, the "shockingly' might not be as mean as it sounded. People in many places in Europe aren't used to seeing people reach obesity weight without any other underlining medical issue. Seeing a member of their family becoming obese would literally be shocking. I understand the heads-up. I don't know if that was obvious but just putting it out there just in case. Still horrible for her to hear.