I actually have felt depressed for a very long time, even in elementary school. And I cant talk to my parents at all, the think that i always am "lying" to them. I dont see anyone that often, and when i do, I'm generally happy around them, as long as i'm not home (so i dont think that people who are my friends will notice a very big change al of a sudden). I have some really clear, clear memmories from 3-5 years ago that I remember were very, very painfull for me at the time, and still are. Im not sure if that might have caused my symptoms.
Everyone goes through weird phases in life, especially when they feel overwhelmed. Obviously what your friend is going through is probably overwhelming to you and often people find it easier when they "zone out" and become numb and try to separate themselves from reality because then the truth does not have to be faced. You may also be suffering from some depression. People who are depressed notice mood change and apathy about things they really cared about at one point in time such as a sport or social events. Did something tragic happen in the past few years? A death in the family? A falling out with someone close? Anything can trigger stress and depression, and it doesn't have to be something big. DId you start a new medication? There are a lot of explinations for what you're going through. Don't automatically assume it's something aweful. As family or friends if they have noticed a personality change (tell them to be honest) and see if they have any idea what might have triggered the change. It might help! Let me know how it goes!