Is this how it works then despite him not causing the fire because it's like a fire starting in a forest sometimes its a freak of nature
ANYONE could claim they didn't start a fire. The court will decide (on the balance of probabilities) whether or not that believe them.
Yes - fires can start spontaneously "in a forest sometimes it's a freak of nature", but this freak occurs in long hot summers, when there has been no rain for weeks, trees and grasses are as dry as tinder. and it is a particularly hot day which causes leaf litter to become increasingly hot, until it combusts, and then the flames spread to other areas, or from the effects of dry lightening - flashes which hit the ground many miles from the storm cloud, and will ignite dry trees and vegetation.
They don't happen in cold, damp weather, particularly if there have been no storms.
I think your "friend" has inadvertently failed to extinguish and cool his weed burner, and it has re-ignited (wood fires are especially prone to pretending that they are out, and then sneakily re-igniting - that is one of the things that makes forest fires so difficult to deal with - they smoulder out of sight and it takes only a tiny breeze/draught to fan them into life again.).
I think you also seem remarkably invested for someone who is no longer with your "friend". In view of the overwhelming opinion on here that he is on a hiding to nothing, I think he would be better to accept liability, apologise, say it was accidental (and I'm sure it was), and pay up.
Alternatively he could go down the nature route, I suppose.
Is there any chance that there is a Phoenix nesting in his garden?