Steve,
My MTH Millennium Mayflower set struggled to pull all 4 cars when I first ran it years ago (I am the original owner of the set). It took me quite a while to figure out the issue. One of the power pick-up rollers on one of the cars was stuck and not rolling. That created enough drag that the motor would not pull the 4 cars. Once that issue was identified and solved, the motor now pulls the 4 cars fine. I would suggest that your friend check all of the wheels and power pick-up rollers to ensure everything is moving properly and also that everything is properly oiled.
NWL