Damn! if i had not got a puppy, i'd have been in Tarifa this week on holiday!
There are a couple of backstreet garages in Tarifa, that always seem busy whenever i'm there.
I don't know the names of them, but can give directions that you can pass on to your friends.
If you come down the steep hill into Tarifa (past the Lidl's supermarket) you hit the traffic lights. on your left, back up the hill by about 80 meters, is a "Taller" (workshop) and they quite often have 4x4's in there. the owner owns the 109" santana that is often parked across the road.
another one:
continue down this road,called Avenida de Andalucia, (towards the sea which you see in front of you), the "main" road take a 90 degree left turn towards the port, but if you continue straight on, then take the first side street on the right (there is a clothes shop and a shop that does paper/pens/office stuff on the RH corner) (and that building is called Andalucia plaza, thats where my flat is in!), down that street on the next LH corner (about 100 metres) is another workshop.
If your friends don't speak Spanish, i have a good friend there who may be able to locate a decent mechanic. His name is Hugo Komari, and he runs the local gym, go back out onto the Avenida de Andalucia, and carry on down the road as if you were doing to head towards the sea, there is a chinese restaurant on the LH side, and about 6 doors down is the "Fitness Estudio de Tarifa", Hugo is a Chillean chap, about 5'4" and musclebound! but he's really helpful.
Failing these options, there are two that i can think of, but involve a bit of travelling:
1) there is a Land Rover agent in Gibraltar. could be costly though
2) that garage that is on Sky TV, called "the garage", based just outside Malaga, all English speaking and english staff (run by a scottish chap).
never been there, only know it from TV:
English Mobile Mechanics
Hope that helps!!!
Jamie.