I received an email from someone who had this problem several years ago. Please, don’t do what he did.
When he was in high school, he had this slightly overweight friend. She was sweet, witty and cool in her own ways. She also had a thing for this guy.
One day he casually said out of mere admiration of her sense of humor “You’d make such a great girlfriend only if you weren’t so fat“. She didn’t snap at him at that moment. But it cut right through her. She absolutely despised him since then as she happened to mention this indicent to a mutual friend. She had body issues and his compliment didn’t help her one bit.
Not surprisingly, she didn’t keep in touch with him after high school. Afterwards, he found her on Instagram and found out she’s married to the man of her dreams who accepted her for what she is.
In conclusion, if you’re ever faced with this problem, it’s better to leave it to her will. If she thinks she needs to lose weight, let it be her own discretion. If she is comfortable in her skin, she’ll find the man who would accept her anyway. Your suggestion may not be of much use, because most likely she’s already aware of that.