I am so angry with his parents?

My partner and his ex have been having arguments lately about visitation over his child, i don't get involved, he hasnt seen his child in over 3 weeks, we visited his parents recently and his child was there, we soon found out that the child had stayed over the night, my partner wasn't informed that she stayed, i was really annoyed with them for not telling him, it feels like .......i may be exaggerating a bit.......but a betrayal from his parents side, they know that he and his ex partner don't get on, so why not tell him that they have organised stay overs for his child with the ex. Would this annoy you???