Look at the remote debugging section in the online help. (Although I don't think the help section makes it very clear.)
Essentially you need to set up SQL Developer debugger to listen and then in the remote code you have to connect to the debugger using dbbms_debug_jdwp.connect_tcp('your sql developer workstation','debugger port'.
If you can't modify the remote code to include a call to dbms_debug_jdwp, then I'm afraid there is nothing you can do.
Edit to add:
Also make sure you have compiled for debug and that your breakpoint is on an executable line (i.e. not a declaration)