How do I tell her to work out just a little?

(my guy friend asked me to ask this because I couldn't give him an answer) his girlfriend is 5'4 and 120 pounds so she's not fat but she has some belly ...nothing major.. just a little... he doesn't want her skinny but he wants her to have a toned stomach.. how does he tell her in a nice way. she's average pretty but she's also very self conscious about it.. she thinks she's ugly sometimes.

Its not a huge issue for him or anything but he just thinks she'd look a lot better and feel better about herself.

btw she thinks she's fat (and she's totally not)
How do I tell her to work out just a little?
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