heavy people sweat more, and usually it is in between the ovelaying rolls of fat that the smell comes from mostly because air cannot get to those places.
There is a high risk of shingles outbreak due to bacteria as well so she should try to do something to lose some of the weight.
That is terrible, and she is comfortable at this weight? I ask because I am about 125 lbs and when I was pregnant I was a whopping 214 lb moo cow due to severe toxemia, so uncomfortable and I couldn't wait to have my daughter so I could get back down to normal size again, not only did I get back to normal size, I went down 2 sizes lower because I was so used to not having sodium "prepacked foods like canned, frozen and boxed".
Maybe you can just hint out a concern to her about her health and heart and how you want her to be around for the kids rather than ask her to lose weight just because she is getting big. See what she says, maybe using health as an excuse to lose weight will make her think twice rather than stir up a dispute.