A lot of people will come across someone who suffers from BPD. Wether they are diagnosed or undiagnosed, a lot of people are at their wits end with what to do when it comes to wanting to help someone with BPD. I hate the term 'dealing' because 'dealing' with a Borderline can come across as they are a burden which isn't the case at all. While the BPD undergoes practicing mindfulness, I think everyone around the person suffering could practice a few steps of their own without automatically thinking they need medication, hospital or a doctor. Even when they are in therapy a lot of traumatising stuff will come up after some sessions. So it always good to be considerate of that instead of saying that they aren't helping themselves when they really are trying to.
I wish I could say an average person usually thinks with logic, but this hasn't always been the case with everyone I have come across in my life. From time to time, humans will react on emotions regardless of the situation. I think what Borderlines long for is someone who can validate their emotions, regardless of how little or big your judgement may think it is. I know the complex and stubbornness someone with BPD can display in certain situations of hurt. But this is where people can practice certain skills to calm them down. First remind them you love them. Then ask them what specific thing has upset them? Listen to them and do not tell them how they should be feeling/acting. Just sit with them through it and remind them that their feelings are valid and you are there to support them.
Following up is essential to reminding the person with BPD that you are here for them. The next day send them a message suggesting to see them and invite them places. When someone with BPD is sick the last thing they need is to be isolated and discriminated against. Obviously make time for yourself but if your usual friends are there regardless of what ever issue someone has with someone (Its usually the person who is ill being singled out) tell your friends you want them there and make them feel loved. As soon as you isolate someone with BPD they start to stress from anxiety and feelings of abandonment. Even if they turn down your offer keep suggesting small things and just listening to them.
Final thing is allowing the person suffering to be themselves. For a BPD person to really want to change they need the positive environment and patience. You cannot rush someones recovery as it will stunt their growth. You are valid to be angry at them but thats where some people go wrong, you have to view it as an illness not a choice in behaviours.