She wants to quit friendship

Hi, I have a friend, who has an overreaction, (I am from Germany), she has also had several problems in her life and because of that she had to go to a therapist for several weeks however she also skipped meetings.
However we wanted to go abroad for 2 months and now she just cancels it due to one stupid incident and our friendship, too!

I hope you can help be because I have no clue on what to do!

So, I have a boyfriend, it's her brother. She always tells me that she has no one, wonders herself why it does not work out and actually does not like to hear about anything. Her brother isn't even allowed to stay at her apartment (mothers apartment) because she gets frustrated when she sees him how we makes himself ready to go or she gets mad when she sees him texting messages to me.

So, that is the reason why I didn't tell her about the day after tomorrow, what me and my boyfriend were doing (just normal stuff, like walking etc.) .( I came home very late and was not able to text her because my flatrate was no activated, so it took until the next morning to get it activated and a few hours later we actually all wanted to meet, me her brother and her)
Outcome > (because I didn't tell her what me and her brother did the day before), she didn't open the door and tells me she doesn't want to be friends anymore) per textmessage.

I really think that is sooo childish! (I didn't tell her she is childish, I just told her her behaviour is inappropriate) She even said she doesn't want to do the work and travel anymore, so i would have to go all by myself.
Can someone please help me on what to do.
I just told her I wanted to meet at a place in a city so that we can talk but I don't know if she will answer.

She says that there is no friendship if I don't tell her things. I only did that in order not to hurt her (see above> reaction brother).
However I have to say it takes always days until she answers a textmessage or email.
i even asked her because she accused me of not telling her why she is not asking anything about us (no questions). Her opinion, I have to do it because "there is something new in my life". ;;;; ok ;;;;;------
We want to meet on Tuesday and talk about it however she said she does not want to change her mind!
She is actually a very nice person but so cold in her mails. She even wrote that our friendship probably was never worth anything if it ends like that! Ha, the problem is I don't want to end it, I even told her that but she want to.

So, I replied that I want to meet her face to face and then she has to tell me because i won't accept it per email.

I need some advices! She does not see any mistakes on her side! I am the bad one in her opinion everything is my mistake!

She thinks I only think about him but not her. That is not true at all, we even meet during the week. But she thinks I have less time and that bothers her. She also wrote in a mail that "people come and go in life". Really.... that is just so childish. I am 21, she is 19.

Personally, I DO think she's being childish, and I'd back off and let her think about things. From the sound of it you're not going to change her mind at this point. She's mad and right now she's just going to stay mad. If you give her time, she may see the error of her ways and come around.

I wouldn't worry about it. I think she'll probably calm down and get in touch with you. Just give her some time.

I wish you the very best. Take care. Hugs, Lee

I asked her to meet on Tuesday. She said we could meet and talk but for sure she would not change her mind. Her emails are very cold and aggressive and it makes me said that she said "if it ends like that there probably has never been a good friendship". I don't think it is fair at all. She has several problems in her life but I feel like her aim "which she has to destroy because of her built up anger".
I don't know if it is right to meet her on Tuesday or if it is too early but when if I change the date she would complain again, either way...
So, how should I go on best on Tuesday?

thank you!