My brother was POA. He had a credit card in his name. The charges he claims were all my mothers. He made payments on this card from a checking account with funds that were entirely my mothers deposits. The checking account had both names on it. My brother had no visible means of income so purchases for food and gas along with cable and phone were all paid by my mother and charged on the credit card.
If mom is in her right mind ( has not been declared incompetent) she can do what she like with her money. But both she and your brother should understand that this "gifting" will make her ineligible for Medicaid assistance if she needs it down the road, either for inhome help or for nursing home care.
Make it clear, also, that when she's penniless from supporting him, you won't be stepping in to provide care, funds or housing.