Advice with VW T5 fault please

[notax]

A good friend of mine is having a nightmare with his 2006 T5 van, fitted with a DSG gearbox. The van is in good condition, with high mikes but has developed a mysterious fault. It runs perfectly 7ntil it gets hot enough for then radiator fan to cut in. At this po8nt it goes into limp mode. This happened after 2.5 miles driving on the road, or 45 minutes if the van is idling on his driveway. He left it with an auto electrician who miss diagnose dethrone problem and ripped him off and he needs some ideas of what could be wrong. He has had some bad luck with his work recently so can’t afford to replace every component until he finds the right one. Can anyone offer any advice? Thanks