Yes, I would step in, because the drinks break is still part of the child’s match environment.
The parent may genuinely think they are helping, but the effect you describe is important: he goes back on checking the touchline instead of playing what he sees.
I’d speak privately and keep it non-confrontational. Explain that the coaches are deliberately trying to reduce instructions and that the break is mainly for the child to recover, not receive a second team talk.
Give the parent something positive to do instead: ask whether he is enjoying it, tell him you liked his effort, help with his drink.
I would not correct the parent in front of the player unless it is unavoidable.
The bigger principle is consistency. If the coach is asking children to make decisions, but a parent gives them six instructions every time they come off, the child is being pulled in two directions.
