This varies with the airport.

The biggest airport in the Pacific Northwest is Seattle. They have a separate car rental facility.

Significant factors are if the passengers are generally slow or if they are able bodied and can walk efficiently. Also significant is if they are using a major car rental company, such as Hertz, Avis, National, or a car rental company that is not in the car rental facility.

Assuming that they get out of the car at a normal speed and the car rental employee meeting the returned car is relatively quick consider the following schedule.

minute 0: car enters the facility
minute 3: car is stopped, all luggage removed, receipt received
minute 6: waiting for a bus
minute 11: bus leaves
minute 23: bus arrives at airport
minute 25: passenger arrives at the check in desks of the airline.

Conclusion: 25 minutes if everything runs smoothly
35 minutes if things are a bit slow but mostly smooth sailing
65 minutes if you have to go to the car rental office, wait in line and discuss something with the agent

If they are a bit slow, have luggage, no TSA Precheck, but don't need to stop at the car rental office and only need to drop the car off with the outdoor car rental agent, then figure on driving into the car rental facility 3 hours ahead of the flight. They will likely have plenty of time to take care of everything but, at worse, will be just on time.

If they walk quickly, have TSA Precheck, no checked baggage, did online check in and like to cut it close, they can drive their car into the car rental facility 65 minutes before departure, arrive at the security checkpoint 35 minutes before departure, and arrive at the gate 25 minutes before departure (while boarding has already begun).

If they want a 99.99% guarantee, then arrive 4 hours before departure. That would allow enough time for a minor car accident but more likely, arriving with plenty of time for a leisurely meal. For most people. 4 hours is unnecessary.