i cant see the problem; PiP is not a means tested benefit, so if she gets that she'll continue to get it. even if she loses her means tested benefits, she'll have lots of money (presumably) from selling her house to live on, an it's not like she'll miss housing benefit as she wont have to pay housing costs.
When her savings do dwindle, she'll be eligible to claim benefits again.
this is how benefits are supposed to work - they are there for people who need them, not people who have money, but when she does nee money again, she can get it again