A friend at work asked if I knew what would cause the locks to cycle randomly on a PT. No remote nearby, and it will lock and unlock while driving as well.
All I could think of was that the RKE module was going south.
Any other ideas or reason it might do this?

PDL switch on the door going south maybe? Joe's (you've met him at the shop) PT ran into a similar issue with driving lights. He replaced the switch and the problem was gone.

They automatically lock at 18 mph but I don't know why they would cycle.

Apparently they lock and unlock at random times by themselves whether sitting or moving. Has no external key lock on the doors, so no way to get in when thay locked with it running and the remote inside, other than thru the rear hatch.
I hoped someone here might have heard of this and have an idea or two I could pass along to him.

Shouldn't he have at least a passenger side door lock? I own two PTs and they both have key locks on the doors. Go here and see if they can help. http://www.ptcrew.com/index.php?msg=1

Strange things happen on a PT when the battery is going bad...might want to check that.