Yes, you may let someone else drive, but only if you’re present in the car, provided that this driver has had a driving licence (valid in Belgium) for at least 2 years. You are also responsible for checking the driving capacities of this person.
Lending your cambio card is not allowed: if you are not present in the car you may not let someone else drive the cambio car. Unless that person is registered as a partner on your contract and has their own ‘partner card’. In this case, you may request the reservation centre to make a reservation to their name.