Well to answer your first question I dont think that someone can be happy and sad at the same time because your friend might be sad deep inside and she is just covering that with happiness . Now to the second question is that she or he is faking it and i mean in a way it is kind of attention seeking but it might be because they dont open up to people and keep it all inside and they think that way meaning depression can cause this and makes them isolate themselves . This is my opinion on your question :)
In my opinion, I think being happy and sad at the same time can depend on what just happened. Maybe its a extremely happy moment, and you start crying. So you are happy and sad at the same times. If your life does not have a extremely happy event, and you get happy and sad at the same time, it is a bit bad.
For the second question, if you tell someone with a valid reasoning, it won't be that you are a attention seeker. If you just told more than one person "I'm happy and sad" you are.
