Can a power of attorney remove large amounts of money from a dementia patient?

Depends what its for. If it's for facility care or medical issues of the individual who granted the authority, then it's legitimate.

More detail would help. Who is this individual, do you suspect financial abuse, and if so, how?

The POA is responsible for paying the bills of the person who assigned it. They r not allowed to withdraw money unless it's t be used for that person. I kept records of everything I spent from Mom's account if ever questioned. A POA cannot use these funds for personal use. If so, I would bring it to the attention of the lawyer who wrote the POA up.

yes