If he's in the US, Domane+ has a custom-for-Trek ECO mode setting that's only 20% of your pedaling input instead of the stock 50%, so he's really only getting a small amount of assist. Would be great when riding in a group since drafting takes the edge off. If he's pulling, the next mode up is a custom Tour mode which is 100% of pedaling input. So that doubles his effort.

I think on ECO mode for this particular Bosch setup, getting 50 miles is no problem at all. Maybe 60-70 if it's not crazy hilly. Maybe more? I have never run it out to the absolute limit.

If the ECO and Tour modes aren't ideal he can get it flashed with the stock Bosch software which is 50% and I think 120% or something, respectively.