I find this a difficult concept to think about in some ways. I mean, if someone is false with you (in the sense that they are dishonest in their intentions or feelings regarding you) then they were never really your friend to begin with, surely? It's a pretty black-and-white thing, at least to my way of thinking. You can't be friends with someone if you're not honest with them. If it's built on a lie, then it isn't friendship and it never was, regardless of what you or the other person thought. It doesn't matter how long the relationship lasts, once it gets revealed for what it is, everything that came before it was clearly a lie, or perceived from an incorrect perspective. There was never a friendship there, so it can't be "fake" because it never existed.
This will be distasteful to consider for some people, but I tend to fault myself rather than others for this because I shouldn't have assumed that these people were my friends in the first place. The assumption was mine, so the fault is mine. Everyone is dishonest. It's just the way humans are: we constantly reinvent ourselves so that we and others will view us from a more desirable perspective. But the most desirable perspective is always our own individual perspective; other people's thoughts and feelings ultimately don't matter to us. We only care about others if it is convenient for us, or benefits us in some fashion - even if that is just a feeling of contentment or happiness derived from the feelings of others...or rather, the reaction we get from others, because nobody can ever know entirely what someone else is feeling. We're selfish creatures. I don't think there is anything wrong with that - after all, if you don't put yourself first, then nobody will, and compassion isn't somehow invalid or negative because you get something from it too - but I can't really blame other people for acting how they wish to act, or thinking how they wish to think, or for otherwise not adhering to my flawed perspective of them. I can't just call them "fake" for not doing what I want them to do. Similarly, I can't just call them "friends" because we have more frequent interaction that I perceive as positive. Could they not turn around and say the same of me, and with equal justification?
I guess this is why I don't consider anyone a friend. The relationships I've had with others have always been built upon mutual convenience and interest, and once that faded, there was nothing left. It was never friendship to begin with, even if it may have felt like it at the time. But that's...OK? You don't need to be friends with someone to appreciate them, and you don't need a lasting relationship (or even a positive one) to have a fulfilling interaction. You can just appreciate people for who they are in the moment. If someone is deliberately manipulative or dishonest...well, does it really matter? You enjoyed the interaction you had with them at the time, and they can't take away the enjoyment you had from that, because you've already had it. All they can do is stop any future positive interaction, so in a way, they're only hurting themselves by doing this. But you used them for your own happiness too, because if you didn't enjoy talking with them then you wouldn't have bothered - you wouldn't think of someone as a friend if you didn't get something from all this as well. We all avoid people we don't like, and we all have sides to us that we would prefer other people not see, or that we try to keep hidden for our own personal advantage, even if that is just for the purpose of portraying ourselves in a positive light.
I would say that if someone you liked suddenly reveals a side of themselves you don't like, that doesn't make them fake. It just means you need to re-evaluate your relationship with them and your perspective of them. I would suggest that it's easier and better to respond with apathy than anger or hurt, because allowing someone who has attempted to invalidate past happiness you have had from them any further emotion or even thought is a waste of your time and a detriment to your mental health. They're not fake friends - they were never friends to begin with. But that's OK, because you had positive interaction with them and enjoyed the time you spent with them, and if they were dishonest or had ulterior motives during that time...well, isn't that their problem?
