My MIL gave her daughter financial authority over her finances. She sends her 600 a month. Is this legal for her to do?

My MIL (96)has been leaving with us for almost 5 years. She used to live with her daughter on and off, and always close to her daughter(AZ). Her daughter complained so much, that we decided to take care of her (FL). It has been a challenge for us since she now has dementia. Her SS check goes to her daughter (she receives around 1,300 a month). My MIL gave her Financial authority over her finances. She sends her 600.00 a month. The questions is: "Is this legal for her to do"? She keeps the rest of the money. Thankfully, we don't need the money and my MIL does not need much.