Handbike rental ?

I'm not aware of anybody in Spain however you could contact somebody like mobilityhire.com in the UK, they offer a triride hire service. If they do not have a branch in Spain perhaps they might have contacts over there.

Have you contacted one of the mobility services (mobilty equipment hire) ?

Alternatively get in touch with some of the larger e-bike rental places, tell them exactly what you want and see if they can assist.

Warning: they're not cheap to hire. In the UK they range between around 150 to 200 pounds per week for short-term hires (under 3 months). Mind you, when you consider how much one of the units costs that's actually pretty reasonable.

I don’t know how much strength your friend has in his arms, but there is someone currently on the Camino using a Grit Chair.