Well I think it is up to the parents to put restrictions on the phone texting...not the grandparents for sure. Who gave him the right to discipline? If it was the parents fine, but to do it just to do it would not go over well in my house.
It was HIS house. That, and she was warned by both her Mom and the grandparents. She had a habit that they knew of before the birthday celebration.
Regardless of the enforcement authority, which is your focus, I find it sad that a family event is of little concern to the children. It pertains to the subject matter of this thread.
It's a delicate thing. If the parents condone rude behavior and the gathering is occurring in their home, then Grandpa overstepped his boundaries. If the parents are indifferent and it was at Grandpa's home, then he has the right to set rules.
It really depends on the relationship. If the Grandparents are generally domineering, then it's one more instance of their sticking their noses in. If the relationship is generally respectful, then they were just being helpful and supportive.
Having been on the other side of the fence, we are careful not to assume authority which is not ours, but we are not just strangers in the relationship either.