It's hard to know what could be behind all this changed personality and the split. You didn't like him from the beginning, did you? What was it about him that you had reservations about? Is it something you are not saying but that she would have been well aware of? So would she feel that his having left would prove you'd been right and she doesn't like being in that position, with the possible discomfort of facing an 'I told you so'?
You described her birthday party events but didn't mention him. Why? Was he with her? Depending on the answer, further questions arise.
She clearly wants to keep some distance from the family about something to do with the split but is it because she feels ashamed of the fact of the split and can't bear to talk about it, or even worse, be asked questions about it or is it because there is an actual secret that has to be kept hidden? Someone has mentioned about being in prison. You haven't said whether he's still a student. Suppose he'd been arrested for something and taken away but it can't be talked about? Or she got drawn into something or discovered something and is frightened of repercussions if she told anyone about it? I fervently hope that she hasn't been given drugs or suffered coercion of any sort.
It could just be an emotional state, though, the shock and upset of a relationship drama and split which is traumatising and destabilising the inner person and their trust in life but the person can't express it or tell anyone, yet may wonder why no one asked her if she's all right or if she is in trouble, of an emotional or mental or some other kind. Because maybe she needs someone to do that and to wrap their arms around her. At the very least, some of the family, individually probably - father, mother, sisters, brothers, should make a point of going to see her, if quarantine rules permit, and if not, at least phoning and talking and expressing warmth and connection, asking how she is and so on and whether she has everything she needs. Why didn't anyone know, is often asked, when a young person was going through a trauma about school or exams or a boyfriend or a job situation. It's important to know, though that's easy to say.