I think that sometimes as with a profligate child, the best love is tough love.
She and her husband are presumably well over 18, ie fully grown-up and they must take the responsibility for their own troubles. They got themselves into this mess, they must get themselves out of it and not look to others to help them.
I know it will not be easy, I would find it very difficult if my sister was in trouble, to leave her to it, but sometimes one must.
If they go bankrupt that will be the end of paying the son's rent, they really will not have the money to do so.
4 Years On…..Health-wise, Has Anything Changed?