gdb carrionfields or whatever the executable is called

attach to the process with 'attach <processnumber>'

from here, resetting the immunity flags is easy, getting a pointer to the character in question is a little bit tricky...

'call (get_char_world(descriptor_list->character, "<NameOfSmugWarrior>"))->imm_flags = (get_char_world(descriptor_list->character, "<NameOfSmugWarrior>"))->imm_flags^<insert value that represents the immune_fire bitvector>'

cont
quit

and then detach the debugger.. (though I wouldn't recommend typing it all in manually, the mud will be frozen until the debugger is detached...)

This method is probably an unnecessarily cumbersome way for somebody with that kind of access to cheat, but I remember it just popped up in my head like 5 years ago when i was playing around with gdb.. i never got to put it into practice though.