You can have something like a nail that will leak when parked a certain way and not when parked in a different position, but it whould not go weeks without losing pressure. I'd change the tire valve seal, which is dirt cheap. I'd put extra pressure in it and look for bubbles in a tank. I doubt if it's a dedicated prankster, but who knows.
Is the tire being checked regularly with a consistent reading gauge? If not, that can be deceiving. A tire can look OK and be losing pressure. Not until it reaches a critical point will the low pressure be obvious to the eye. so if its not being checked regularly it could appear to be going down quickly.
Your friend could try a different tire shop.