how is your house set up. can you just go outside while she naps? bring the monitor? when she wakes bring the kids in and grab her to head back out (with the warming weather this won't be so hard). If you are comfortable with this tell the parents and give them the option of no nap or she naps while you are outside.

My daughter is the same...an amazing sleeper but NEVER sleeps in the car or stroller. Never has. At home you can plop her in her bed any time and she'll happily curl up and put herself to sleep.