A while ago on another site I had an interesting discussion on exactly this topic. I find it amazing how many good programmers have all sorts of uses for interactive debuggers - but say that debugging is not one of them! It certainly isn't for me. And in this problem in particular, the code you are describing sounds like stuff which I would suggest that you not use a debugger on.

So how would I approach this problem?

First of all I would use strict. If his code cannot readily be cleaned up to pass that, then he is likely doing some nasty stuff. I would prefer to rewrite than maintain dynamic symbolic references.

Otherwise I would then step back and set out to understand the code on a conceptual level. It will take a while. But that is the main problem with debuggers - they make you think about problems at too low a level.

Now you say you cannot check his code out. Do you have a development environment that is separate from production? If so then develop in that. If not then tell the rest of us where you work so we know to avoid the company, then try to figure out how you can set up a small test environment.

If you can't do that, then read the code anyways. And let this be a lesson for you in the future. Don't use stuff that you don't really understand which you are not completely confident about... :-)