All parts of the pipe should be solidly attached. With something like
bayonet joint or at least (3) sheet metal screws at each slip joint.
Because things happen, and house fire should not. That simple.
Strap bracing may also be advisable. Think in terms of having a raging
chimney-fire going, with white-hot pipe roaring like a jet-engine. (As
I've heard it described.) Under such conditions, everything should stay
together, and threaten nothing. (I'm implying also that, given the
picture so far of shoddy workmanship, _you_ should check all clearances
from flammables, too. That includes lumber behind sheetrock.) Think 3'
between uninsulated pipe and flammables.
In fact, you might be well-served by having your fire marshall check
out the installation. Often it's cheaper to do it right once- certainly