how do i go about telling her i think she has BPD?
What do hope to achieve by telling her she has something she cant control, probably not ready to accept, let alone ready to undergo difficult and extensive therapy to try and reverse?
What do you think the difference will be the day after you have effectively told an emotionally unstable person who is high reactive to criticism, that everything that has gone wrong in the life is "her fault"?
The normal recommendation is that it is unwise and counter productive.
So what to do?
Start off by managing your side of the relationship and how YOU interact with her. Along with the impact of how her actions impact on you. The aim is first to reduce your stress, while cutting the avenue of her using you as a channel to deal with her issues, via projection. If you can cut the current avenue for blaming their issues on others they are left holding them. Only then are they are in a position where they are open to seeking healthier ways to deal with them.
In short, until they are actually in the right mental space to want to seek help with their issues there is no point highlighting them. Leave any diagnosis or explaining of the problem to the therapists, it will only be heard as a damning accusation from you no matter how carefully you word it.
It is also a lot easier to work on your own way of dealing with the RS, if you can go about it undisturbed. You need to get your side rock solid first, as even if they are open to it they will be struggling with it.
As a side note even though my partner has spent most of her life acknowledging she has a mental illness, finally accepting she has BPD and there is no quick fix, along with the realisation of her part in the dramas, resulted in depression and hopelessness. This started a path of endless ODs. Currently we have had 18 trips to ER in last 4 months. That is not unusual