In general decreasing the operating frequency of a power transformer is a bad idea as they become less efficient and generate more heat. Too much heat equals a dead transformer.

I don't know if the transformer in the 975 is beefy enough to take the extra heat and I wouldn't try it on my own unit.

Also I don't know if Outlaw uses the 60Hz directly for anything.

If your friend really wants a 975 then he should look at an inverter that will change 50Hz to 60Hz.