It's not discrimination if...

it's justified.

Vilify a black guy for his skin:
a) is it changeable? No.
b) could it have been avoided? No.
c) is it harming him or anyone else? No.
d) is it actually bad? No.

Vilify a fat guy for his fat:
a) is it changeable? Yes.
b) could it have been avoided? Yes.
c) is it harming him or anyone else? Yes.
d) is it actually bad? Yes.

Should you say something? Maybe.

Loved ones want to protect those they love. The family of the grossly obese will cocoon their dangerously overweight son, tend to him. Support is promised if ever action is taken, but in the eternal meantime, they tend to him. And even should those loved ones speak the horrible truth, and say to this fat man he must change, those words carry no threat -- for no matter what, his loved ones will tend to him.

It's the opinions of strangers that matter; an acquaintance not a lifelong friend. One unloving truth spoken earnestly will carry the endorsements of years of unkind eyes, and can excoriate the tenuous veil of denial that perpetrates such hopeless flagellation.