Do the CDT portion from Salida to Twin Lakes. Lovely stretch of trail, relatively easy logistically (Drop off at Monarch Pass, pick up on HWY 82 by Twin Lakes).
~80-85 miles.
After you are done, you can easily go into Leadville for a well deserved meal.
Coming from the Denver metro area, Leadville is really easy to get to as well. The Leadville Hostel is really hiker friendly if you need to layover a night.
If you want to make it easy on your BIL, there is a bus from Leadville to Copper Mtn (off I70) daily IIRC.