Maybe a little on the pricey side for a Sigma prime, but the USB port is intriguing. It'd be cool if it solved some of the AF issues without having to send the lens in for service. Is this feature new? Haven't seen it before.
I think it's better to get the camera he wants and not waste money on another APS-C in between. Though I do agree he should do both glass and body at the same time. A 6D w/kit lens (24-105L) would be good start. If you can, wait until the price comes down after initial release.