Nathan Frechen via OBRA
So he's got two main options - either east then south, or south then east. He'll have to avoid the Warm Springs reservation.
East then south - out 26, through Government Camp, down Barlow Trail to Tygh Valley, and hook up with the Outback Route, and divert into Bend.
http://ridewithgps.com/trips/4316799
South then east - last summer I made it down to Oakridge staying off pavement as much as possible. Out 224 to Ripplebrook Ranger Station, then down to Detroit via logging roads that paralleled 46.
http://ridewithgps.com/routes/6673075
http://ridewithgps.com/routes/4631102
and then cut over via Rt 20, hook up with the Old Santiam Wagon Rd into Sisters - http://ridewithgps.com/trips/3667454
Both of those options would highly favor a MTB rather than a cross bike, especially if loaded with touring gear. The Old Santiam Wagon Road was very soft in spots, even with a 29x3.0 tire.
The south then east route would be way more climbing. I'd recommend the east, then south route unless he's specifically looking for the harder way to go about it.
If he has any additional questions, feel free to extend my email.