I would never have done it. I don't think it's acceptable to leave an infant overnight without there being some sort of emergency. I believe it to be traumatizing, even on a subconscious level. If a baby is removed from its mother for a long period and doesn't seem to be suffering, I would question that child's attachment to its mother. But that's just me...
I think the problem with this is that we (I assume) are lay people interpreting scholarship on attachment theory. I have read that separation anxiety is a good sign - considered part of healthy development. That makes sense to me. But, when we talk about this -- I wonder if we are not looking at the bigger picture. Some people are suggesting that leaving a child over night at all in infancy/early childhood can be damaging. For the record, I would not choose to leave my infant for a week. But, I don't think the scholarship backs up some of what has been suggested - that is is less than ideal for a child to be cared for by people other than her/his parents, that a child who seems OK over night w/o a parent is not attached and etc. We know that we want infants to develop secure attachment(s) to caregivers.
Talking about what is "ideal" is sort of tough because that's pretty subjective and I doubt that any of us are living this ideal life. When I read some of the posts here I will admit that the parenting philosophy that focuses really strongly on just a mother/child bond and the nuclear family -- that, to me, does not sound ideal.
So, to me, leaving an infant (almost toddler, really) with a grandparent for a week does not feel ideal -- neither does the scenario where the child has no other attachment outside of the parent unit. And that may well be where the parents in the OP found themselves. So the OP family may well have felt like they were finding the balance between two less-than-ideal situations. I think we've all been there.