"Look, Mum, here's the deal. Fridays I need some time to do X, Y, and Z. It's a lot easier to do them by myself so I can spend more quality time with O when he is here. These chores won't do themselves, and nobody else comes in to do them, so it's not an option, plus O enjoys being with his own mates."
If your mum wants to volunteer to do those things for you. having heard your stance, then yippee. If not, she has no business making those parenting decisions for you or pressuring you that way IMO.