Up sh*t creek...

Based off what I have seen in the houses from the 40s, there is a good chance that the line has is trying to get the plug out is going to go right back and connect under the toilet. The direction of the plug should give an idea of what direction the line is running... if it doesn't turn. As Bob said, I have also found the kitchen line was tied in direct to the main line outside the house.

Chances are it is all cast iron under the house but if it isn't then he could put his snake in and then use a metal detector to trace it.

Owning a bunch of rentals over the years, I have ran in to all kinds of situations that was less than pleasant to deal with.