Ceuta customs can occasionally be awkward about 3rd party owned vehicles. Make your friend's letter as official-looking as possible, translate it into French and take a duplicate. Keep it simple, OTOH something like:
I_____the registered owner of______give full authorisation for ______ to temporarily import _____into________between the dates of____
You get the idea. Add respectful cordialities and include dates of travel, full names, addresses, registration, chassi number, type/colour, marque, passport nos. etc.
For a single vehicle that you are driving that should work fine.