Unless you've tested OP's fiancee's system yourself and found that to be the case, I wouldn't be so sure.

I had a Logitech G500 with awful coil whine and the opposite problem: it would stop whining when moving and start when idle. I suspect it had to do with the power saving mode that lots of mice have, where the laser power supply ramps down to dim the laser illumination after a period of no detected motion.

A lot of times I've noticed it's the USB controller. It's quite evident when you have headphone ports near USB ports.

I can be tabbed out and hear when I get into the plane in PUBG from the increase in whine. It's quite nice actually.

Not necessarily. Possible, but I had a cheap HP mouse that whined briefly when it first came out of sleep mode. Couldn't have been the computer, since it would make the same noise when the PC was turned off.

That is not the case - the mouse itself is whining, it does it even when the receiver is not plugged into anything.