Sadly I think unless the children are capable of looking after themselves and have found a place where they can live without her comfortably she has to stay with them; I think it might be the law.
It's a shame she feels this way but instead of trying to get her out take holidays with her so she'll enjoy it more. It will make her happy, keep the kid's mother in the country and help you to feel happier about her situation.
However, if this doesn't work out I'd think she'd have to find somewhere for the kid's to stay and take a break then maybe go back in a while.