Before he even gets to that, why not figure out what the load from the
washer and the treadmill are together. There may not even be a
problem. Or alternatively, don't use the treadmill when the washer is
running.

Short of that, I agree that any stepdown device is going to be quite
expensive to have enough capacity to run a treadmill. The easy and
correct solution is to just run another outlet. Since it's a basement,
that should be pretty easy and/or reasonable to have done.