OK, let me chime in from the OTHER side as a babysitter....
I babysit my grand full time -- and NAP time is not set in stone for a CHILD. it is child driven, not grandma driven, or mom or dad.
SO....if either Mom or Dad texted me and said they were coming early to pick up the child, I'd text them back and say "Chloe is zonked ... not a good idea." Afternoons are Chloe driven. I can lay her down at 1....she talks until 2, and then zonks out. OR she can zonk out right away.
If you gave me 20 minutes notice that you were on your way, and she's napping, I too would say "she's napping, so stick to your normal time".... If she's had a good nap and is still sleeping when Mom or Dad pick her up, then waking her is not a huge issue for the evening. BUT if she didn't go to sleep until later, waking her at pick up time is a bear.
Heck, my ringer goes OFF at 1 pm until she gets picked up so she doesn't HEAR it .... the house is on full "silence" mode.