Unless I've missed something, it didn't say she was found fit. The article doesn't mention why her benefits were stopped, it could be for various reasons. Being found fit wouldn't stop PIP necessarily as PIP is a non means tested benefit and can be received even if you are working full time.
It is a fucking tragedy but to blame DWP is, in my opinion, simplifying a complex problem and not doing any good. Instead of always casting DWP as a villainous entity, out to murder as many people as possible, we should be looking at the entire picture and looking for ways to solve the issues. For instance, the family could have, in one of the good periods, gone with her to the various services (DWP, housing, council) or written a letter with her detailing her condition, how it may affect her returning forms (for example), giving permission for her mother to be allowed to discuss the claim etc. Maybe it's this, advertising that this type of thing is an option,that we need to focus on promoting so other people are less likely to fall through the cracks.
Unfortunately, those who work in departments such as housing and DWP (who are not psychopaths despite the "diagnosis" by a p.p.) have to work within the law which is agreed by the government. As much as staff and managers may bend the rules as much as possible, there are things they cannot do and rather than blaming an entity, it is because of those that set the legislation.
It is a horrible tragedy and my heart goes out to the deceased.