How lovely the grandparents, who live far away, get to see their grandchild on a regular basis. Isn't that what many of you complain about not happening in your lives; you can't win with you lot. I was a SAHM for many years, by choice, and I was always flitting off to spend days at my parents' house, as we had moved out of the area where we grew up for my husband's job.
My parents loved it and it meant they had a close relationship with their grandchild in the precious few years they had with her before they died. Other than travel costs, she is probably saving her husband money on heating and other utility costs, food, leisure activities etc. while she is away. If it is causing a problem in the relationship it is their business to sort out, no one else's.