It's very normal for young people to do this on an adhoc basis in return for some riding- most places I've know of try to offer a ride a day, but this isn't formalised and wouldn't always be a full hour lesson- and may not always be possible. If she did two days and got a hour of riding with instruction, I would see this as reasonably fair. If it's the first time she'd done this, it's very likely she got some informal stable management teaching too.
I think in some set ups, it can be really exploitative but in others it's a great way for kids to get experience of being around horses and what working on a yard is like- as well as usually getting in a bit of free riding. Longer term, it also helps keep the cost of lessons down for everybody.
It's also worth being aware that the yard obviously can't pay her under minimum wage, so they can't just "chuck her a tenner"- and there are all sorts of considerations involved in taking on staff. That's why being paid in kind through riding etc is so much more common. As she gets older, it may well lead into a chance of paid employment (or at least a good reference) if that's what she wants to do.
As long as she is safe and happy, I'd let her carry on doing it, but if she ever wants to stop then equally, I'd let her do that to (although not let down the yard owner at very short notice).
FWIW, I do agree that it's not really a sustainable business model for the industry long term- but I also don't know what the solution is. Without this kind of free labour, a lot of riding schools would really struggle to make the books balance, and then this in turn means that riding as a hobby becomes out of reach of so many people. It's also hard to find staff to work on an adhoc basis or for just one day a week (often a Saturday or a Sunday)- so without this type of thing going on, you'd definitely see riding schools have to close.