If someone is overweight/obese and care about their health, they'll work to better themselves.
It can be hard to tell a friend, especially if it's a girl.. But it's better they hear it from someone that cares then a random stranger who is out to make her feel bad.
Eventually she'll come to her own (hopefully).
If you do decide to help her straighten out her life make sure she knows that exercise alone doesn't help, and neither does diet alone. It needs to be a fine balance of both.