Here is my view: 
      I can not say that drinking coffee by itself will derail your weight loss, but it can contribute to it.. There are many 'Rules' to successful post surgical weight loss. Individually breaking one or another once in a while will not harm you, but that is a slippery slope. First you break one once in a while, then more often, sooner or later you will find yourself complaining of how weight loss surgery failed you. How often as dieters did we say, "Oh its OK; I can eat this today. I am starting my diet tomorrow?"That was my life. for all of my obese life. Which has been most of my life.
     So often when people fail, remember this is not a cure all, they never looked at what they were doing. When asked, "Did you exercise", they say "No". "Do you eat all your proteins?" "No." Did you take your vitamins?" "No?' 
     This surgery is a tool. Work it and it will serve you well; neglect it and you will fail. I was a two 8 pack a day Diet Coke drinker before my surgery. I stopped drinking cola and other caffeine drinks the month before my surgery. Instead of a can in my hand. every waking hour of the day. I now carry a water bottle. I found that the people with the most success followed the guidelines the strictest.
      I figure I have gone to the extreme and had my plumbing altered. I have not come this far to fail! I have lost 220 lbs so far. I am 15 months out, and I can now, most likely, eat many things I could not eat early out. I don't and won't try them! I don't want to know that I can have them. I am just fine without the things that made me fat in the first place. This journey is about changing habits and getting healthy. I have lived well for 15 months eating healthier. I now have no excuse as to why I can't continue eating this way for the rest of my healthy life.

Nick