A friend you're close enough to that you can talk about poop -- having to poop, the smell of it, that you just fired off a big one, that sort of thing. Very valuable.
"Ugh, I still feel like crap. I fired one off 10 minutes ago and it didn't help."
'What, a poop?'
"Yeah... we're poop friends, right? I can tell you that, right?"