ss formulated on Thursday :
Ok here some images of the caps, back of board and front of board. The cap
wont pull out but then it may have welded when it exploded.

https://imgur.com/vCG2M8O

https://imgur.com/KhigJP1

https://imgur.com/71nQ3Pa

A much better idea of what we are looking at and it is a switch mode
type. The fuse is on the mains input side. If that is really intended
to be a fuse-holder, it is not one I have ever come across before. I
cannot see any obvious signs of damage, apart from the seriously
damaged fuse. Are there any obvious blast marks on the earthed casing
etc.?

I suspect there is no fault, apart from the obvious one, that your FIL
was messing about with it live and managed to short the fuse to earth/
metal casing. With that sort of blasted fuse, if there were a fault on
the PCB, the likelihood is that those diodes would also be well blasted
too - they are not.