I got my Argentina pilot's license at Morón (suburb of Buenos Aires) at a flight school called FlightCenter. But if you're interested in visiting La Cumbre, Cordoba, there is a cool flight school there called AeroAtelier. You can do a "vuelo bautismo" (first flight) or several flights, or get your license there. The scenery is much nicer than Buenos Aires. The guy who runs it is from Switzerland and they speak English.
However if you're living in BsAs and he wants to pursue his license, a local flight school would be best. Most of the instructors will speak English, since they are almost all going for their commercial/ATP license, and they must speak English for their ATP (airline transport pilot). However, at most of the airports, and to pass a checkride, he will have to speak enough Spanish to safely communicate on the radio. For me it was a great motivator to improve my Spanish. The air traffic controllers at Morón, for example, will not speak English.
Since he has a UK license, he could in theory "convalidate" it. I already had my US license when I did my license in Argentina, but I opted to just do a new, separate license in Argentina, because I needed to learn the local rules, and practice/improve my Spanish on the radio.
Flying in Argentina was very rewarding for me... I have a few videos of some flights here on youtube. Here's one from Ushuaia to San Fernando, Buenos Aires: