Very likely his leaks are at or towards the tail end.  Rust always tends to
succeed in perforation there first.  The answer is a qualified "yes" on the
pipe and muffler.  Don't ask me how I know.  I've never tried an extremely
high heat application.  Application problems:  With holes in the pipe, the
weld tends to fall inside when you put it on.  Hence, you tend to glob it
on ever thicker.  It falls away and is wasteful.  It is illegal in the
State of Georgia to waste JB Weld in this manner.  Trick:  Tape over the
hole just enough to cover it, or stuff as small a piece of paper in there
as you can to cover it.  Apply the weld right over it, and completely cover
the hole and tape or paper like you are cementing over it.  Also, wire
brush the area before starting.  The weld needs   fairly clean metal to
bond well.  Smear enough around the area to give it somewhere to hold
onto.  When it dries, it will harden to a solid covering.  Hole gone.  This
lasted a long time on some other cars.
HK
Weldmaster, $CRAP