As a teacher, I've never needed FT care in the school holidays, but every CM has asked me to pay 50% of fee as a retainer.
If I wanted to use any of the days, then I would let her know in advance (say I wanted to do some decorating or get some school work done or anything that would be a lot easier without dc), and pay the full cost for that day. tbh, if she wanted to have someone else on those days, it wouldn't bother me, but it might be quite difficult to find someone who could fit around my days, as I retained 'first shout' at any days. (Some people send their child 1 day a week, so they are still used to going and don't have to do settling in stuff in Sept, for example).
If the CM doesn't take a retainer, then the affect that has is that her hourly term time rate goes up, to cover the fact she has no income for those weeks. Your friend has to look at it as what it is costing her over the whole year, not just that she seems to be paying for "nothing" in the holidays.
Even if the CM doesn't take on anyone else, I think it only fair that there is some agreed notice period if your friend wants to use her on those days, as she might just be arranging trips out, or even a nice lie-in, or, to be fair, doing her own decorating whilst she is not expecting her mindees.