I've been in the uncontrolable point of sickness due to alcohol a couple of times myself, I came to the conclusion it all matters on WHAT and WHEN you've ate and how fast you drink. I have drank a good amount over a long period (5-7 hours) before whilst clubbing (10-15 alcohopops\pints of beer)or at home for that matter, I am only 17 and I weigh 11 stone with a small-medium build. Before I have been really in the mood for drinking, drank 4 glasses of vodka within a hour and spent the next 3 hours around the toilet, ill, before I finally stagger to bed and waking up realising it was a shit night. Other times I have drank 6 or 7 glasses of vodka over 5-6 hours or more and been fine. Also eating food with high carbohydrates helps alot, also word is that milk helps, it lines your stomach, although personally I've never tried it.
The brown shit will just be what was mixed with whatever you drank or what he ate earlier on that day. Nothing to worry about. Take that from experience .