A close friend taught me invaluable lessons on how to interact with people who have borderline mental instability:

Keep your communication objective and polite. Don't provoke them into any kind of reaction

They behave they do because of the surge and swing of emotions. Don't be judgemental. 

Listen to them with empathy.  You don't have to agree with everything they say.

Most importantly, don't pester them with questions because it pushes them off whatever stability they have been able to make for themselves.

Finally, don't get emotionally close to them