As it is a trike there may be some possibilities.
In the UK bike engined 'kit cars' are very popular. As a result there are several conversion boxes that are available that mount after the bike gearbox and before the differential. One of these could well be a possibility but the costs may be off putting. IIRC it is about $450-500 for the reverse box and then you would need 2 bespoke propshafts. Depends how much he wants reverse.
