If you’re not driving a truck, stay on I-70 over the Front Range until you get about 100 miles past Grand Junction and follow US-191 to I-15. Depending on where you’re going in Idaho, either stay on I-15 or fork off via I-84. Way prettier than Kansas, Eastern Colorado, and Eastern Wyoming, but a bit hairy for trailers at some points.
Yep, it does pretty much depend on where in Idaho he’s going to end up. Mine was a south-to-north route; his may well be a north-to-south. I nearly ended up routing through New Mexico on that run. It’s a really, really big country.
That is a fun way. Probably not good for Ohio-Utah, unless far Southern Utah is the destination.