Grandmother who just doesn't care?

Our household does not eat at fast food restaurants. My 7year old son knows it's because the food is garbage. He recently stayed with his dad and while his dad works, grandma keeps him. Grandma (who is diabetic) also had her other two grandchildren, who are both obese and eat whatever they want. When they announced to my son that they are going to Taco Bell my son told them he is not supposed to eat there. I got a phone call twice this week asking if it was okay to take him to McDonalds or Taco Bell. The first time I said no and they took him behind my back (he soaked the grease off the chicken biscuit with a napkin and ate it). Today I get the Taco Bell call. I have explained to grandma that this is a lifestyle choice. So today I got her to give him a sandwich instead, but then she took him and the other two to Taco Bell and they ate that crap in front of him!
How would you handle this situation? Should I rip her a new one?

I let him eat freshly prepared foods, vegetables, fruit, ice cream, ect. We don't buy or make fried anything. Things without preservatives, the occasional soda (like one every 2 weeks). My son is fine with what we give him. He doesn't beg to go to these places.

She's old. But you should stress to her how you want your child to be raised and she needs to respect it. Your the mother, she's not. Be respectful though. My mother would just take a little more time out in the morning to prepare our lunches according to our dietary needs. Maybe you need to spend some time doing the same. The other kids eating it has nothing to do with you and your child. Let their parents deal with juvenile diabetes and obesity. You continue to take good care of your child. You also need to address it with the father. He also needs to stress it with the grandmother. I'm pretty sure that with the both of you on her case, she'll get it together.

I do want to give you a little praise for not allowing your child to eat fast food. I've met many mothers who says they won't allow there children to eat that stuff but never stick to it. Great job, two thumbs up.

she should be more respectful !! but since he doesn't eat there regularly.... mabe a little allowance wouldn't hurt much !! best wishes !! you don't want your son to feel the odd man out all the time !!

so wut do u let ur son eat???