I started to make a list but then I realized that this isn't the first time this question has been asked. It's pretty easy to just google stuff but here's a partial list of some of the things out there:
https://www.verywellmind.com/how-to-help-a-friend-with-bpd-425221
https://www.helpguide.org/articles/mental-disorders/helping-someone-with-borderline-personality-disorder.htm
https://themighty.com/2017/10/borderline-personality-disorder-friendship-tips/
https://blogs.psychcentral.com/caregivers/2017/09/15-things-not-to-do-with-someone-with-borderline-personality/
https://everydayfeminism.com/2016/09/support-friend-with-borderline/
https://www.bpdcentral.com/borderline-disorder/bpd-relationships/
But I also wrote down a list - it might be interesting to add items and maybe upvote/downvote somehow to figure out what the most important ones are if there's a way to do that. I also don't claim to know what I'm doing and I'm probably going to get a bunch of stuff wrong, but it will be helpful to get that pointed out.
In no particular order, probably some are more relevant than others…
1. Maybe ask yourself why you want to be their friend.Its probably okay if you arent sure but consider some of the following:
* If you want to be their friend because you think they can be easily manipulated, then you are a piece of shit
* If you want to be their friend so that you can save someone, then maybe you should ask why you feel the need to do that first.Maybe its a way to feel a sense of self-worth or something, but just make sure youre not using them for your own personal fulfillment.
* They are not a puzzle to be figured out for your own entertainment.Maybe there are some aspects of them that you will never understand and you should probably be okay with just accepting that.
* Yeah – probably others
2. Make sure you can tell the difference between they are just casually chatting and when they are really asking for support
3. When providing support, try to determine if they are just venting and need someone to listen, or if they want help figuring stuff out.If they arent looking for help and thats all youre giving them, then its going to be pretty frustrating.
4. Sometimes you might understand how they feel, but sometimes you probably dont.I dont think its constructive to try to prove that you understand them because thats pretty pointless and counterproductive.
5. Although sharing your own experiences with them to show that they arent alone might seem like a good idea, it also kind of invalidates their attempt to express how they feel by making it about you, not them.
6. Just like with any mental, physical, or emotional disorder, dont blame whatever issues come up as just being a consequence of it. It totally invalidates how they feel and theyre not wrong to feel that way.
But I also wish making this list was easier - it just seems sooo inadequate and I still feel miserable. I could have done better.