My first question is 'Why?'.
What is your student's reason for wanting to quit? Does she simply not want to spend her summer taking lessons? Does she find practicing too tedious to continue? Are the lessons getting in the way of a larger goal? Does she not know?
If her reasons are immature such as 'I just don't want to play this summer' or 'I don't know', encourage her parents to continue the lessons. However, if it is something she has thought about, for example: 'I want to play softball and I can't while I'm taking lessons', it means that, yes, she may enjoy piano, but she has other priorities.
Before you can make any responsible choice, it is necessary to understand how serious she is about quitting. Sometimes, even with something enjoyable, there's an impatient itch to change the pattern. It's the 'seven year itch' of hobbies, as it were, and in that case, hauling through is best, but you cannot assume her reasons are insubstantial until you ask.