Firstly well done for reaching out, it’s wonderful that you are seeking help!
Have you spoken to her and tried to find out what seems to be the main cause of her being angry?/ lonely?
(is it social problems/ side effects of medication/ current lockdown etc)
Second- what is her age?
I’m a 25-year-old female and I have ADD, it can sometimes be very lonely for me too. It might help if she could have someone of similar age with ADD/ADHD to speak too? I am happy to be that person if she wanted too. I know when I get angry, it helps if everyone around me stays patient. My mum would still call to check in on me when I was at university and that helped me. A call a day, even if not much was said told me she was there for me.
Try to bond over interest/hobbies of hers this can do wonders. Does she like books/ particular music/ places etc or discover a new one together!
I hope this helps (This is my first reply in a forum so sorry if it’s too long)