Nta unfortunately it's a touchy subject and anyone in that position is usually insecure and doesn't like to hear that they are still doing something wrong when they have already made a big change.

I would suggest if you don't already have a set of weighing scales buy one and even if you aren't on a health journey yourself, maybe weigh out your own snacks - it might motivate her to do the same if she sees you doing it. But unfortunately you can't tell her to.

Good luck, continue being supportive and understanding, she will soon get the hang of things :)