Old post, but willing to chime in.
Good idea, Linket.
We are at the stage where we always get stray friends or BF/GF coming along for a family event. I don't know how many Thanksgiving dinners we have to pull up extra chairs. This isn't such a problem at home.
But at a restaurant, one would hope they would have gotten permission to bring someone along, but it doesn't always happen. Parents should teach their children better. The grandmother should not have to handle this kind of stress.The parent can pay for the daughter's extra person. Put this back on the parent's shoulders.