I have a rubber mat under my treadmill workstation over hardwood. I did not put anything down underneath the rubberized mat. The rubber should prevent scratching of the hardwood.

Most commercial gyms are likely placing the the rubber tiles over concrete, or if you had it in the basement.

So, no, I would not use a rug pad.

You mentioned weights, how much weight and is he really lifting or just flinging around some dumbbells? Oly bar and rack, or a weight machine with pulleys?

I would be concerned about the weights (because they'll be picked up and put down often) but not the treadmill. My treadmill is sitting right on my hardwood floor, which is solid oak that was refinished just about two years ago. We recently moved the treadmill forward to put some shelves behind it, so I had a chance to see the floor underneath, and it wasn't scratched at all.

If you plan on heavy weights or a free weight rack, you might want to consider a separate weight platform. You can Google for DIY or else purchase one.