Hiya Cris, Can't really give you any advice - not sure about the Piriton, this would only stop itching possibly but not presumably a cure? There are lots of great people here who maybe can help more. Unfortunately vets are pricey, would maybe your friend being a pensioner be able to go to the PDSA? Some vets will let people pay in installments, I know mine will..
I hope the cat gets sorted, poor thing