No matter anyway I have ever tried to mention to a female they are putting on weight it has gone horribly ( daughter, spouse, GF, random friend ).
Now males it has been fine.
"Hey Fred put down that extra chicken wing, ya growing ******"
It has always been when they finally click the switch in their own head and then you can help them.
Which sucks because stopping it from happening is way easy from fixing it after it has already happened.
Good luck....