My mom is on Medicaid, and needs to do another spend down. Can she pay me back for the rent I paid for her last month?

First, no Mom can't pay you back. Second, does Medicaid know you are paying rent? Where u paying it to Dad? Now things have changed with Dad's death you need to run this by Mom's Medicaid caseworker. Renting the house will effect her income and you need to be upfront.

If mom writes a check to you it will likely be looked at by Medicaid as "gifting". Which isn't allowed & mom would get a penalty for the amount of gifting.

If it's that you paid a mo or two of her AL rent, I'd suggest you ask business office if mom can write checks to the AL for the amount you paid & then AL sends you a check for the "overage" or "refund".