No-one here is making a buck out of this or 'likes' the logical conclusions.
I'm also an engineer and would have quite a few questions for your friend.
Steel is weakened by heat, but that's the reason why concrete or 'chartek' fireproofing is usually applied.
It takes a lot of heat input and a lot of time for heat to overcome the fireproofing and weaken the steel.
This is why I can't imagine how a couple of tanks of jet fuel could weaken the entire WTC structures, vaporise the aircraft, cause the explosions in the basement, and leave molten steel in the wreckage two weeks later.... it just doesn't have enough heat input to do all of that and in any case most of it can be seen burning up in a fireball in the first couple of seconds after impact.
