This reminded me of a question they asked me when I applied to work retail at VS:
You are the only worker on staff and the store is busy. You are trying to ring up a woman at the register. Suddenly, the phone rings: you answer, but continue to scan the woman's things. The woman on the phone asks you to do a price check on an item that is not within reach, but is very expensive (more than what the woman at the counter is buying). She says she wants to purchase it later. You are the only one in the store still, and a line is forming behind the woman at the register. Who do you help first?
The correct answer was the woman at the register, because she was there, in person, and paying. Who knows if that woman on the phone would have come in to buy that item? If you didn't deal with the customers in front of you, they would leave and you would lose 'guaranteed' money.
Granted, your situation was a bit more complicated than that. I agree with you in that it seemed pretty lose-lose. The mother doesn't want to come from the beginning (which is odd if she's the one paying all this money for her daughter to cheer, yet doesn't bring her to her classes?), you're being pulled in two directions, BOTH of which involve 'guaranteed' money. Yes, you were payed to work with her personally, but you DID get a coach she also works with personally and loves, and it was during warm-ups. It might have been a bit different if it were a coach she's never worked with before. I don't know if there IS a right answer for this one, because who's to say you wouldn't have gotten in trouble with your boss for ignoring the other parent and they left? On the other hand, if the other people who wanted to pay got angry and left, it might have just shown your boss that you REALLY need someone at the front desk because he's going to lose business.