What would you do if you were me?
During an airline safety briefing, the flight attendants always say: Be sure to secure your own oxygen mask before helping anyone else with theirs. It's important to bring that same mindset into the rest of your life.
The situation you described appears hazardous to your mental wellbeing and quality of life. You've already voiced your disapproval with this person's actions to no avail. Any further attempts to correct behavior of another person are unlikely to make a difference and will only lead to resentment. Based on what you've said, it seems as if the best course of action is to phase this person out of your life. If you maintain a relationship in any way, this person will continue to drain your energy at best and potentially drag you into a dark place at worst. The good thing is that you are separated geographically -- that makes this sort of extrication much easier. You can just start to ghost those text messages, replying less and less until you just fade away. This person will move on to someone else when you stop giving the attention she is craving.
I've lamented many times in the past on this site (it comes up often among the sorts of people that are featured here) the sorry state of mental healthcare in the present day. It's sad and a little scary that it's possible for someone to gradually lose their mind, eventually reach the point where their brain is so damaged that they themselves can't recognize that they need help, and wind up simply self-destructing.
It's one of my biggest personal fears, in fact. But when it comes down to it, the system is not set up for regular people to intervene. It's sad because this person clearly has some pretty severe psychological issues that need to be addressed, but any non-professional who might step in and attempt to assist will just wind up being dragged down and damaged in the process. There is, unfortunately, nothing that a regular person can do for someone like that.
I realize this is really easy for me to say not being part of this. It's hard to phase someone out of your life -- especially when that person had previously been close to you, and especially when that person remains close with other close friends of yours. It'll be a challenge to cut out this person while maintaining a relationship with your best friend. Just remember what drew you together in the first place. If the friendship is important to both of you, you'll find a way to maintain it while you cut ties with the other one.
What you should do for your best friend is a trickier proposition, but sometimes you just have to remind yourself that he is also an adult who can make his own decisions. You can't control what he does or how he reacts to situations., and openly criticizing someone else is unlikely to do much good. You simply pulling away may be enough to inspire your best friend to do the same if he shares your frustrations with this person's behavior.
Wishing you the best of luck. Stay strong.
