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It was a new area of work he hasn't done before. Just over three weeks into this (a week ago) there was a fire at his house.
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He is contacting the LL tomorrow.
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Wait, what? Why not call/e-mail him the first instance he knew about the fire?
1) I doesn't matter whether your ex realised the terms of the LL insurance or if the LL had an insurance at all. He should check his tenancy agreement which more than likely have a clause stipulating he has to inform the LL if the property will be unoccupied for longer than (usually) 2-3 weeks. He clearly failed to do so, not only that he still hasn't informed the LL about the fire!!
2) Although the fire might not be your ex fault failing to notify the LL about the vacancy and subsequently about the fire itself is negligence and might make him liable for damages. You said the the smouldering has been ongoing for days, if he was at the property it could have been put out in time before the fire broke and limit the damage.
Trying to shift the blame to faulty electrics or fire alarm not working (completely irrelevant since he wasn't there to hear it in the first place) doesn't change facts that according to your investigation they are apparently legal. And the Fire brigade report doesn't point fingers at them.
3) It beggars believe that we are finding about this on the MSE forums before the LL. Even if the tenancy agreement have provisions making the LL liable to provide alternative accommodations at his cost in case of unihabitability of the property, I'm fairly sure that your ex negligence in this case will render the tenancy agreement void. So he better start looking for new home, likely kiss his deposit goodbye and pray it stops there.