Posted: 06/11/2017 at 19:02
I would buy in some bags of well rotted farmyard manure from a garden centre ... it's not expensive and will be free of weedseeds, which isn't always the case with manure direct from stables.
I would also add some Fish Blood & Bone which is a slow release organic fertiliser, also obtainable in packs from garden centres. ... use it before sowing ieach spring according to the directions on the pack and that should really revitalise your father's veg patch.
Best of luck to him and his veggies