Is it really your job to tell this girl? I mean, you're obviously very concerned for her well-being and health. But, she's still young... maybe it's best to try to hang out with her more, and by hang out, I mean be active... take some walks or something.
Because I don't think saying "You're fat, and you'll die" is the way to come across to a 12 year old
First of all, I remember that at this age, long term affects were NOT on my mind. Secondly, I would be thinking "Who the **** do you think you are talking to me? You don't even know me!" And all that good pre-teen stuff.
Approaching her parents could pose to be tricky. On one hand, you're deeply concerned, on the other, they could see it as you trying to tell them how to raise their child. (And insulting them by saying they haven't taken care of her health.)
Still... perhaps approaching them is best, because the only way a 12 year old is eating entire boxes of pastries is if their parents bring the boxes home, and then allow the child to eat them all. If you approach with true concern in your voice, and do not put blame on them (even if they are at "fault"), they may listen.
The last option is the "Is it really any of your business" option. How well do you know these people? If your mom gave you a call to tell you about it, I'm not sure that you're in regular contact with them. Can you imagine them approaching you about a delicate subject like this if your child were overweight? It may come down to a "noneuvmybizness" thing in the end. And if you have to, you can tell your mother that as well.