Option 1 would be to modify the source code of the calling app and insert tee into the output pipeline to get a copy of the output for review at that stage.
Option 2 would be to write a wrapper script around the executable in question. A quick script that passes on stdin and arguments to the real app, then tee's the output to a location for you to review and also spits it back out the same way the app would should be just a couple lines to whip up. Put it someplace special and make add that location to the front of your PATH variable, then run your application.
#!/bin/sh
cat - | /path/to/realapp $@ | tee /tmp/debug_output
