BS”D
Nope. What the grandmother did was pure rishus. Not big time stuff but rishus just the same. Limud zechus – maybe she is troubled or ill and not a reshois, but still this should not happen.
I had something even worse happen where the relatives of a mentally disabled man plopped him down in my expensive MBC concert seat as I had been slightly delayed, and then disappeared. The man did not understand English, Yiddish or Ivrit (or was told not to respond). Worse yet, my back was strained and there were no other seats; otherwise I might have been OK with dancing in the aisles for a bit before taking care of the issue. Finally, I found the man’s relatives and demanded that he be removed.
I told the relatives that they were turning a tohor (the man is not mechuyav in mitzvos and is considered on a higher level) into a gonif. They did not even react or apologize.
Sometimes, rishus is just that. There are times when we must say “shygetz aross”!
Even if the granddaughter were chas vesholom disabled I would expect the woman to ask the first girl or her parents to give up the seat. You don’t touch someone else’s kids except to help out, period.
I don’t know what I would have done had the relatives of this man asked me to trade seats after the fact (probably would have refused as it was geneiva any way you slice it but they did not even offer me another seat and I have no idea what the story was as this was way back and security may have been loose enough to let someone sneak in).