What if a person not of sound mind is tricked into signing a power of attorney what happens then?

First, the trickster should be reported to the district attorney for taking advantage of a person who is at a disadvantage. Perhaps the DA could provide advice as to what steps should be taken to counteract the false POA. Perhaps they could draft a notice that could be served on any institution where the POA could be used to access bank accounts or investment accounts, or sign social security checks. If the victim owns real estate the notice of the false POA should be recorded in the land records. The legally incapacitated person needs to have a guardian appointed by the court. Someone needs to commence that process.