oil pressure drop/car stalls

The oil pressure gauge is mostly a dummy gauge on our cars. Once the gauge reaches a certain point, it doesn't go up or down much if at all. Once pressure drops below a certain point, the gauge drops.

Since the oil pump is run directly off of the distributor, a correct oil pressure gauge would show rising pressure as the RPM's went up. This worries the average consumer, so Ford put in what amounts to a dummy gauge.

I think you're running into the reverse problem - your daughter's Mustang isn't idling properly. When the engine dies, the gauge drops to 0. I'd recommend resetting the idle using these instructions.

The IAC motor could also be bad. It's the big black sensor on the side of the throttle body; it lets the engine idle when there's a drain on the motor at idle, such as running the A/C. Reset the idle first, then you can test the IAC.