Bkd engine swap

[Stehad]

Hi all please could someone give me some advice? I have a mk5 golf gt sport 2.0 tdi on a 58 plate,my turbo went last month and damaged the engine,I’ve bought another engine from a Audi A3.My mechanic friend has changed the engine and it’s starts a little reluctantly and ticks over fine,but when you apply revs the oil pressure light and stop light comes on.Hes changed the pressure switch with my old one and put a pressure gauge on and it’s running 4 bar,the Audi engine had the loom cut out roughly on removal so he has used my golf one,is there any difference in looms?
He said he thinks the engine is fine it could be an electrical or sensor issue? He has a few faults on his snap on diagnostic tool,egr is one of them.Please does anyone have any advice?