Need advice for my friend with BPD and a romantic relationship

My friend sent messages to her boyfriend the past two days and each time he opened the message but didn't respond. She went from happy two days ago to despondent right now.

She realizes, on some level, that her mind is being irrational. She mentioned to me she was like an animal without "object permanence." The animal forgets that objects not seen still exist. She said it was like this way for her and love.

She also stated that she thinks her boyfriend hates her, she doesn't want to bother me, her friend anymore, because she sees herself as a burden, and she says she hates everybody now (which I know is not true because she doesn't hate me, obviously).

She is getting therapy right now but does not see her therapist very often.

What advice would you give her to how to approach her boyfriend? How can she deal with the fact that he doesn't respond to her every day, because sometimes he is busy?

Thank you in advance for all your help!