I suspect he has had an algae problem. Since he has been treating the fuel since owning the boat I think the black in the filters if fine particulate are dead bugs. Anything slimy or stringy is obvious however. To visibly turn filters black in only 60 hours of use wouldn't be asphaltenes right - especially if he is using fuel (boating)? I'd still be shocking the fuel.Hold on.
Is it black slimy and/or stringy or is it like a light oily dust or powdery?
If the former , shock with Bio-Bor and treat with Diesel Kleen green label Run the fuel out and repeat.
However, if it’s a light and dusty it may be as Asphaltenes which are a byproduct of being heated and pressurized in the injection system. A little bit of that is normal.
Post some pictures. If you’ve been following the “cocktail” it likely isn’t a biological infection.