My guess is that you have a few things going on here and the big one is and inadequate use of flux..........or of using the wrong type of flux.
The blackened head tube on the inside of the joint is very telling. It looks as if there was no flux on the parts (or flux was of the wrong temp range) and if this was the case the brass would not bond properly to the base material and the strength of the joint would be seriously compromised.
Ideally you would see bare and clean metal in all the areas where you applied heat due to the cleaning action of the hot flux. This would allow the brass to properly 'wet out' on the base material and form a strong bond. Without proper flux the filler would not wet out and the bond would be very weak.
One can lay fillets without any flux at all but it requires that the base materials be very, very clean and that the person using the torch is extremely skilled. If no flux is used the heat needs to be kept as low as possible to get the filler material to flow and if it gets much hotter one can get the blackening seen in the photos.
I think that even if the fillets were larger that the results would be much the same and it would have still failed.
I hope no one was hurt and that the builder gives it another go. I would urge them to do some practice joints and work toward having no blackened base material.
I hope that helps -
Dave