I see what Lathyrus is getting at though.
Someone changing the parameters of an arrangement to suit themselves, whether by bringing a dog, a child or a husband to an arranged outing, can't expect others to fall in line just because she wants to do it that way. We don't know whether that was the case, but if so, I think it was rude to bring the dog, however much he is loved by the OP.
I would have asked if it was ok to bring the dog before agreeing to go, and would have asked if others were happy to eat in a dog-friendly area before expecting it of them. If I'd been there and someone was eating on her own because of the dog I'd probably have gone to sit with her, but would have felt that my experience of a group lunch was being watered down a bit. I'm a dog lover though, and am not allergic or afraid of them - if someone had brought a pet spider she'd be on her own
.