One of my friends has gained at least 50 pounds in the past couple years. She always says she's "working out" but the scale doesn't lie

Every time we hangout, she wants to go get food. Nothing wrong with that, however she always chooses the WORST thing on the menu while I'm eating chicken and veggies. I tried going on a short 1 mile walk and we had to turn around because she couldn't make it...
The worst part of it, is that she claims she's on this diet where basically she doesn't eat for 3/4 of the day then BINGES the last part -THAT ISN'T HEALTHY and it's only hurting her metabolism.
Her sisters are the same way. They all don't do any physical activity.
I feel like as her best friend, now is a good time to say "hey, you really need to start watching what you're eating and exercising" without being mean. She overall just is gaining at a rapid speed and I know she's going to go at it with me but a good friend would say something.