[Seagondollar]

It turns out the problem "miraculously" cured itself. Herself said in the evening: "I'm going to try it again.", it worked, and she's been driving it since. (!!???!!?!?!) I do hate intermittent problems. At least those that might reoccur at an inopportune time.

 

[ImperialCrown]

Make her aware that there is an emergency Park latch release in the side of the shifter console so that no one gets stranded if it does occur again.
Park override details should be in the owners manual.

 

[Cornupenuria]

Make sure the lubricant hasn't all gotten wiped off the plunger in the switch itself. That sometimes happens in hot weather. Just a touch of white lithium grease should do if you can't move the existing grease around with a finger tip. Sometimes the plunger will push out far enough to activate the brake light contacts but not the transmission lock-out contacts.