Morning Carol Welcome to the forum.
I am the same age group as your Dad, remember we can't stop getting older but who said we have to grow up??
A loss of power can be many things apart from the DPF, blocked fuel filter, failing fuel pump, throttle body fault, leaky or faulty injectors the list is long.
I don't drive a diesel but have son's and a daughter who do, what I have learned is the DPF (diesel particulate filter) is pretty much indestructible. Yes they clog especially when used for short journeys but there is a regeneration program built in which can be triggered by a good knowledgeable garage with the correct software. The regen programs can also be triggered by driving the car, a good blast in 3rd gear to keep engine revs at about 3k (sons Toyota pickup) this cleans the DPF you can also get them chemically cleaned talk to a diesel specialist.
I would check that the specialist he has talked to has actually read the codes? A blocked DPF will certainly trigger lights and engine codes plus they can't be reset if there are other associated faults such as EGR (exhaust gas re-circulation) blocked, injector or glow plug faults.
I would be talking to a different garage, check out your area for a good diesel specialist or Landrover specialist (same engine in the Range Rover) also google DPF regen and read up.