What is a true friend?

So, I was always under the impression that a true friend was someone that you could be honest with and they would understand, especially when you are trying to help. Apparently I misread something along the way. I think that this time I've just had enough and knowing that I've done nothing wrong I have no remorse or guilt for this rift in a friendship. I feel that if I can't be honest with someone I've known my entire life then what is the point of our friendship? I offered not to drink so I could take her out to have fun and get her mind of things that were bothering her, so when I said we should keep that topic out of our conversation and she continued to talk about all the things that were going wrong, I told her it was turning the mood sour and I was under the impression she didn't want to thing about that stuff, hence why we went out in the first place. Apparently that makes me rude.I had a long week as well and really needed to get out and have some fun but, if she needed to talk fine, I was there for her. Just not at a bar.. I asked her if she wanted to go home and talk about it and she got really upset with me. Basically I'm a bitch for not wanting to sit at a bar and watch my friend get drunk and make a scene about her problems, when everyone was staring and being rude. But since I offered to go back home and talk, I'm the rude one. Somehow that just doesn't add up to me... I'm tired of covering for her lies and being there for her when the one time I want to go have fun too and not talk about what is wrong in the world I get accused of being a lame friend and ruining her night. I thought that I was doing the right thing, what a REAL friend would do. I guess not. What else do you do in that situation? Let them act like a drunken fool and have people give ugly glares and sit back and do nothing? I'm sad that something so ridiculous as a guy she's been seeing for less than a year can cause so long of a friendship to rift and this time, I'm over it. Who needs a friend that will act like that when you try to help them? Not me. I have enough going on that I don't need added drama of who she said what to, and keeping try to cover for her incase someone confronts her. Forget that. I'm done.