The likelyhood of a bottle failing as you mentioned is around slim and none...
More likely is the result of flame impingement on the container containing a liquified gas... This is referred to as a BLEVE Boiling Liquid Expanding Vapor Explosion.
http://gaussling.wordpress.com/2007/04/29/bleve-boiling-liquid-expanding-vapor-explosion/ The container heated as a result of either the fire of the structure or a fire from a leaking torch. The liquid in the tank is heated resulting in an increased vapor pressure that may occur faster than relief devices can control, at the same time the steel container is heated to a point where steel becomes weak. At this point the tank fails and the gas in the tank is released...
Boom!
Here is a video of one on a grand scale!
You need to review this individuals safety attitude, and determine if it is indeed a thing where he has become increasingly reckless in his attitude and behaviour or if it is a freak of chance. Any body can have a bad day! But If he's inviting them then something needs to be done.
A review of the safety requirements for soldering operations should be done.
1 Require a servicable fire extinguisher to be on hand at the actual site of the soldering work. Out in the truck is too far away!
2 Water spray bottle on hand and used to soak materials that can present a problem
3 Flame protective Mat on hand and used when needed. Many plumbers use an old piece of smoke pipe. This is unacceptable as the galv. steel pipe conducts the heat right through to the other side while the flame protector does not.
4 Cool Gel offers phenominal protection against heat for surrounding materials. I remember when the stuff was first introduced The manufacturers rep at a supply house demo day sprayed a $100 bill with it then invited one of us to "torch" the bill. I believe everyone that saw that bought some.
5 Review the need to check for a draft ino a wall opening or up a stud bay and how dangerous it can be.
6 There is also a heat stopper putty that is available. This will stop heat from conducting down a pipe and can be used to block openings.
7 Call your insurance carrier and ask for help from their loss prevention staff in providing training. They came to our shop and set up life fire training in the use of fire extinguisters... No Charge!
With all this available I'm not going to say it never should happen but the rate should be much lower than it is...
There really is no good excuse these days for charred wood near a sweat joint or, a fire at a jobsite.