My mum gave up work when she had me and my sister until we went to school. We had very little money growing up and had to have hand me downs & didn't have the luxuries kids have these days.
If I ever have kids, I intend to do the same thing - give up work and make sacrifices - too many of my friends have kids and stick them straight into nurseries as babies so that they can continue their career, and to cut costs on the childcare, pass them off to grandparents for a couple of days a week too. I just don't see the point in having kids if your not going to spend the time looking after them IMO.
My parents are 65/ 66 respectively and I wouldn't expect them to look after my children for me on a regular basis - they would be exhausted and it's not their 'job' to do so - it's expected more and more these days for grandparents to take on the role of part-time childcare assistants and it doesn't sit well with me, especially if they are struggling to cope with it.
I hope your friend enjoys it, I really do, but if she is finding it difficult, she must speak up and ask the parents to take back the responsibility - they are their children after all!