shertex,
before you go planning on fixing the problem, please get a blocked filter, cut it open with an angle grinder or hacksaw, post a pic of what you find inside it.
I doubt the mech who has advised you friend knows anything about diesels.

There is a good chance that all you will need to do is remove & clean the strainer, drain the tank, put some clean fresh diesel with bio kleen in the tank & get them to only buy fuel at a station with a high throughput in the future.
We wont know though until we see what is in the filter.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving