I'm not buying the ex girlfriend "just friends" either but I'm also not buying the "talk loudly to be heard" and "hate arguing" parts...
She would have no problem hanging out with both her besties (her girlfriend and YOU) if there's really nothing to worry about and talking loudly to be heard and flipping out and yelling stupid things when frustrated/upset are very different definitions and if you hate arguing, why keep arguing when it clearly is not working or doing any good?
My bpd girlfriend loved to argue and I would catch her smiling when she would see me get frustrated and "talk loudly". At first I thought what a childish beyatch but no way was she going to win by my losing my cool (cuz I'm cool,) so didn't say I saw her smiling, didn't react, I didn't fall for that again and I didn't keep pissing up a rope, meaning I don't keep doing what doesn't work and most of all I won't let her get under my skin or yell or react to deliberate childish bs games like that. It was (and sometimes still) a power struggle but my boundaries are firm and I will call her or anyone else out immediately when they are tested and I will do this without losing my cool (or leave before it get any further) and this includes lies, sarcasm, insults or attacks/questions to my character etc...
This can be exhausting with a bpd, especially MY bpd and I can only control me, not her, these are my boundaries, not hers and constantly testing boundaries and daily accusations no matter how well or nicely put are crossing my boundaries and I am not putting up with or signing up for this kind of life with her. So here I am. I trusted her until I found out the extent of the mean, crazy, weird things she was doing. She makes no apologies, owns nothing, has no remorse and does all the things she is so afraid of being done to her, the exact opposite of what she should be doing. Now I know what bpd is and I tried to help her, tried to draw lines a and maintain boundaries but now i also know I can only help myself and she is not going to change (for the better) so I'm out...
I see echos of these kind of things in your short post and I am sure you can see where I'm going with this. i have nothing to gain here and im not selling you anything so i hope you can finish the last sentence for yourself...