Visiting day-guests are allowed to park for free. They should let the security gatehouse know why they are coming, and possibly give the details of the guests that they are visiting.
Technically they are not allowed to leave their car at any WDW resort and then plan to go by bus to one the theme parks (it would be a method of avoiding the theme park parking fees) but how is anyone going to check where you are with the guests that you are visiting? And POFQ has plenty of parking space, and isn't over-utilised like the monorail resorts.
Andre