My future son-in-law is trying to fix an F150 truck. With the engine on, he can't get the gear out of park. What does he do to fix it?

Two possible problems. The park pawl inside the transmission is stuck or broken. A similar situation can be experienced if one parks on an incline and places the shift lever in park. If the car rolls back or forward, even a little, it can jam the pawl. Try having someone rock the vehicle back and forth, it may come out of gear. The next problem is the shift interlock switch is inoperable. On vehicles with a center console, there will be a one inch square plate near the park position on the drivers side of the shift lever. Remove the place and press the button, it should release the shifter. On a vehicle with the shift on the column, it is usually under the steering wheel on the column. The owners manual for the vehicle should indicate exactly where.

Thank you both so much. After I asked the question, I kept reading carguru answers and one person said it was related to the brake light, so I tested the brake light and indeed it doesn't work. How does that additional info change or not your answers?

Without a wiring diagram in my hand for your particular vehicle, allow me to add. The shift interlock switch on the vehicle may be powered through the tail light switch. If the switch is defective, then the interlock switch is not receiving power when the brake is pressed. On all newer vehicles, the brake must be pushed to remove the shift lever from the park position. If the tail lights do not work, then power is not getting to the bulbs and I suspect the switch is defective.

All said and done, search the net for a video of manually overriding the lock on your year, make, and model truck. Normally, the override is on or beneath a cover if you have a console shift. If on the column, it is usually underneath the steering wheel, also beneath a cover.