I have four children. Three of them are normal-weight or under-weight. My middle son (about 8) is overweight. He eats the same food as the rest of the kids, is limited on junk food, goes to gymnastics, and runs around like a madman every afternoon with the kids in the neighborhood (jumping on our trampoline, swimming in the neighbor's pool, riding scooters/bikes).
Some people just carry more weight. At one point, my eldest started to tip the scale a little, and then he shot up, seemingly overnight, and has remained very thin, since.
I would recommend to the mom (only if she asked!!!) :
1) Make sure he's getting at least 1-2+ hours of active playtime/exercise every day.
2) Get him involved in sports.
3) Limit sugary drinks, including juice, to 1-2 per day and drink mostly water/decaf tea/unsweetened drinks.
4) Keep fruits and veggie snacks available
5) Eat well-balanced meals (with dessert limited to 1-2 meals in a week)
6) Make sure he's getting enough rest.
If she doesn't ask, I'd invite him along to do active things and try to help him develop a love of activity.
I can tell you though, my middle son gets probably a solid 3+ hours of high intensity exercise every day and he's still "husky" at the moment. And we do all 6 of the things I listed above. Our doctor isn't worried, because he knows he's very active. He may be 95th percentile for weight, but he's also 95th percentile for height.
Try not to judge, some kids/people are naturally chubby, and some kids just have this as part of their growth pattern.
Our insurance company sends us letters telling us how to eat healthy and get our son to lose weight. My eldest saw that and said, "You'd think they could look at the rest of us (kids) and know we don't have a problem with that!"
And the doctor, whom I've asked many times, isn't worried either.
Again, there are a lot of body types out there. More important than looks are healthy habits including good diet and exercise