It sounds like your friend has dealt a cruel blow there, mate.
Having a house damaged caused by the soot in the chimney catching fire, is a common occurence where I come from and I expect it happens in other places as well.
But, to have a Fire Marshall come up with something like that, is wierd to say the least!.
There is a clear-cut line between a Deliberately-Lit fire and an Accidental fire.
And I reckon that the Fire Marshall is just being spitefull.
Where is the evidence of an Accelerant?
What time of the day did this occur?, Do you know what day it is?, would be the questions I would be asking of the Fire Marshall.
Arson is a very tough call and one that I would not use as a Fire Officer, until such time as a decent investigation had been performed, as in this case though, it doesn't sound like this has been done.
Just as a final note, where I can't find the cause of a fire, I usually get other people in to have a look as well, for thier opinion, I'm all for honest decisions on fire causes, especially where it concerns Insurance claims, either way!.