I would tell him to avoid Windows 10 unless he absolutely needs it.
For ST6 there is no need for W10.
Biggest mistake I made on my home computer was 'upgrading' from W7 to W10.
in my experience ST7 with the latest MP is "the oldest" version you can sucessfully use on Windows 10.
Anything older (ST6, ST5 etc) will not work on Windows 10, altought they can be instaled on this OS.
Solid Edge is crashing at the startup so I'm assuming it's installed.
Anyways, the reason for crashing is possibly that ST6 is not supported on this OS. You can try running this in comptibility mode for earlier versions of windows and see if it works out or not (try windows 7 or 8).
Officially speaking, it's supported upto 8 only. See the attached excel file below. That will solve all the compatibility questions your customer might have.