Why would you stay with a psycho girlfriend/boyfriend?

Yeah, you read it right, psycho is the right word and I am not misusing it. To say that my friend's girlfriend is "toxic" would not do it justice. Reality surpasses fiction, and this is the clear example.

A member of my squad has been in a catastrophic relationship the past couple of years. His girlfriend claims to have premenstrual dysphoric disorder, and uses that as an excuse to walk all over him. She is a psychiatrist, the irony, but behind the curtains she abuses our friend and who knowns how many people more. They break up almost every three days, hell, she wished my friend's parents deaths throughout the COVID crisis after they got ill (she never apologized). She calls him lazy, the man works all day and has helped her multiple times economically. They have planned in having children (Oh help me Lord) and a whole future if they get married. Here is the deal, the degree of mental abuse puts her right up there with Cthulhu contesting for the ability of driving someone insane.

We have told him, not once, not twice, not three times, but at least six times a month for 2 years that he needs to move on from her. The guy is not stupid, he knows and he is fed up with her. He cheats on her, she probably cheats on him,, but they are still a couple. The one day she suggested she kissed another man made him extremely jealous and mad, he even cried in front of us.

I have tried to get in my friends head, why the hell is he there? I asked him the other day if she had suicidal tendencies. He responded that indeed she is suicidal, this lead me to think that perhaps if the only man that is willing to tolerate her bs and is willing to have a future with her... walks away and abandons her... it must be a terrible weight on his shoulders. But maybe I am wrong or are too young to comprehend. Really I know I should not give a damn about other people's problems but it hurts to see someone that I known for like 8 years in a state of total misery.

Please help me understand.
Why would you stay with a psycho girlfriend/boyfriend?
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