Videos taken with Sony A5100

lisapea A mac is NOT the solution to most problems. It is simply an operating system some people prefer. You can have this exact same issue with an older mac that is too short on rom/ram for today's camera needs.

How is he doing the file transfer from camera to computer?

If his card reader is too old the card reader may be the problem. Being an older computer it may be the processing speed of the system is slow.. not the fact it is a pc, just the fact different processors have different speeds. By upgrading your old pc to windows 10 it may be w10 is taking up too much memory which slows down the computer. One of the frustrating things about the digital world is usually everything needs upgraded together especially if the gap of the older computer and the camera is 5 years or more.