I feel that it is unnescasary to bring this to the forums. I fail to see why we are avoiding such a person when we know so very little about the situation. There are no chatlogs present, and no real indication as to whether or not this person had rage-quit, or if they simply disconnected before getting the chance to answer. This is the sort of stuff that results in bad communities.
Well, kicking them without saying anything if you see them is in poor taste. However, going "I saw a screenshot on the forums where you rage quit. Was it true?" and waiting for an explanation; I don't see anything wrong with that.
Most likely the person will immediately leave the game or start mouthing off to you. That's when you know it's true. But perhaps it's not and they will explain as such.