It can and does happen.
I knew of an elderly lady, resident in a nursing home because her children worked abroad; they held POW and visited as frequently as possible.
A close friend (whom I did know) appointed herself unofficially as Carer and gradually insinuated herself in the Nursing home, visiting all day, every day. Gradually she prevented other friends from visiting, claiming the patient did not want visits, began supervising and criticising the way the staff treated her, then tried to persuade her to change Nursing Homes.
At this point other friends managed to make contact with the adult children because they were concerned about not visiting the patient; the children checked with the Home, discovered what had been happening and put a stop to it.
Both were horrified because they knew and trusted the Friend but had never appointed her as Carer; she was doing it through (her own) best intentions, no financial incentive, although it transpired there was a strong element of control involved in the relationship previously, before the lady was ill.
The 'Carer' still cannot understand why the adult children were horrified, because' I was doing it all for (Name).'
I do believe it she did it with the best of intentions, but no concept of how far she had overstepped the boundaries.