What I would do in this case it tell the parents how you feel about waking children and what you are observing when you wake their child. Remind them that sleep begets sleep and cutting naps short with a child that struggles with bed at home usually just leads to a sleep deprived child. Sleep is crucial for brain development and growth and behavior.
Then tell them you will do are they request and wake the child at 3pm (no earlier). Tell them you want them to touch base with you in a week as to how it is going at home. If the child proceeds to show signs that they need more sleep with you and things have not improved at home then you will resume longer naps.
Hopefully this is enough to show them you are willing to try...and for them to see that shorter naps likely isn't helping on their ends.
I have found that pointing this out to parents and them seeing a shorter nap didn't magically help means they are more willing to resume longer naps. Another thing i have found useful is to inquire about is whether the child's evenings are better on weekends when they control the naps. I have had parents truly believe that my shortening the child's nap would mean easy nights then when i ask if they nap long on weekends (never as they only sleep when rocked) and if their evenings are better the parents suddenly realize crap no weekend nights suck just as much! They then usually tell me to let their child sleep as much as they will with me because they clearly need it.
I have worked with many, many children that nap till 4pm that still go down no problem at 7-7:30. If they have good sleep habits at home and are used to it it does not impact their ability to go down easy.
The easiest thing on your end is to get the child down earlier. Does she sleep in a different room than the others? Can you pop her in bed right after she eats lunch then finish up with the nap routine of the others to buy her extra time?
All things aside...I am with you on this. You don't wake a sleeping child (especially a child that wakes miserable and that sleeps poorly at home) and 3-4hr naps are perfectly fine and sleeping till 4pm is perfectly fine BUT this isn't your child so aside from standing by your set minimum nap time (typically 2 or 2.5hrs) you should honor their wishes of trying a shorter nap. BUT make sure they understand you will wake at 3 but won't start waking at 2:30 next week and 2pm the week after. Try to get them to see what you are seeing on your end and to give them the chance to see if a shorter nap magically resolves things on their end. If it does then the child should soon adapt to a 3pm waking as they will be getting that much more sleep at home. But odds are they won't see a change and will move on to blaming the next thing they can for the poor sleep habits.