Yeah, while the High Peak / Monsal are admirably 'bentable, the surface is mostly hardpack, and a bit too rough for prolonged riding on a non-suspended trike.
The Bristol-Bath path would be better - flatter and a tarmac surface - but only outside of commuter hours. It gets quite busy at certain times of day, and a trike would mean having to give way on the narrower bits.
I reckon tactical use of the Redways / MK Bowl is the best approach, if only because it's much closer to home should things go wrong. A quiet housing or industrial estate could also work, once he has a bit more confidence with the trike.