best advice i have from personal experience hope it helps also assuming its a girl lmao
1. tell her to relax people forget to
2.tell her in kind and calm way that she needs a counselor it doesn't make her mad or less then anyone it just means a helping hand and they can help her sort out her feelings and support her.They can help improve social skills and help you see a better perspective if she's afraid of the stigma tell her its nobodies buisness she should try it talk out her feelings to a pro
3. it 100% helps to know that there are others feeling the same way tell her that it isnt uncommon to not have friends many many MANY people dont have any she should wait she'll find a good one eventually in the meantime if she attends school or collage make some good talk to people just hanging out not really friends but acquaintances
advice for you : it is not your job to take on her feelings ive made big mistakes like that costed alot dont become her outlet it exhausts you it is not your job nor do you know how to deal with her feelings correctly instead direct her to a professional

so my childhood best friend chatted me on messenger yesterday that she felt like she didnt have a friend, is never anyone's first choice and shes tired of interacting with everyone. so i told her that she is her mother's and my first choice because she is. and she just seened my message in messenger(we're long distance right now) and i know that its her depression that causes her to push people away but its starting to piss me off and wander if i should continue to force myself on her because its staring to depress me as well. because i dont wanna lost a friend. what are your advice?