I think the story is...

Your friends asked grandparents to take care of young kids and stayed over the visa allowed time. Now the parents get used to life here and are attached to the kids, so they don't want to go back as they won't be able to get the visa after staying here illegally. My suggestion to your friends, get citizenship and apply green card for the parents. So they can travel freely between China and US. They might like staying in China longer than in US.

What I don't understand is how "6年花掉了十幾萬美元"? Do they have their own apartment/house, own car? It is almost $2000 per month expense; not easy to spend that much unless have separate household. Of course, unless the grandparents only use luxury brand products...