My blunt assessment of the young woman is that she has Borderline Personality Disorder (BPD). Here are its official (DSM) clinical signs:

(1) frantic efforts to avoid real or imagined abandonment;
(2) a pattern of unstable and intense interpersonal relationships;
(3) identity disturbance;
(4) impulsivity in at least two areas that are self–damaging;
(5) recurrent suicidal behavior, suicidal gestures, threats or self-mutilating
behavior;
(6) affective instability;
(7) chronic feelings of emptiness;
(8) inappropriate, intense anger;
(9) transient stress-related paranoid ideation or severe dissociative symptoms.

I know about BPD because I used to be an Admin of a now-defunct board which supported people who self-injure (cut themselves without suicidal intent) - this is part of criterion (5).

Usually somebody needs five of those signs to get that Dx but from what you have said she seems to have all of them.

People with BPD are very manipulative, as is reflected in your experience. For your own well-being (all that I am concerned with) please have nothing to do with her.

If you want to you could pass on the BPD list to DB as a "heads-up". letting him know what he is getting into with the young woman. Your call - whatever is best for you is the only thing that I am worried about.