Any teacher's I have pay full-time from Sept - June. I usually fill the spot in summer with another child so the parent doesn't have to pay but in the event I can't fill July/August then the parent is required to pay full-time even if their child is away.
Also when I had kindergarten children who were here every 2nd day (and were paying a minimum of 5 days in a 2 week cycle) the families were required to pay full-time for summer.
Now that mom is full-time I would switch her back to full-time. If she is off during the summer then she should still pay for a full-time spot unless you can fill it with another child.
I think sometimes we get caught up doing favours for people but in the end we are the one's who do without. I say you gave her a break and now she needs to get on board like the rest of the full-times.