Worried about my friend, what should I do?

My friend (who lives abroad, this makes it a bit harder to judge, hence my question here) has been with a guy for 5 years now. The first two years they seemed ok, but since then she broke up with him (and got back together again) many times. According to her because “he just won’t let her go”. She would say repeatedly say that “for her it’s over, no matter how hard he tries”. That she calls him her ex, but he keeps calling her his girlfriend. Eventually they always slide back into the relationship, until the next ‘cycle’ or ‘episode’. She once even broke up and moved to a different city, alone, (after they had lived together for a while at her place in a smaller town). But even then they would still end up being together eventually, she saying he ‘convinced himself the city could be ok’ and stay with her a few days a week (his job is still in the smaller town). The keep going in the on/off cycle for another six months until at some point her complaints became a bit more worrying. She would say he doesn’t show her any physical affection anymore, despite saying that he still loves her. That this makes her feel insecure, unattractive and rejected. That she already has low self esteem and that this makes her even change the way she holds herself around other people. And that she feels she’s to blame for it. That they basically live like brother and sister and for her it’s over, but again “he won’t let her go”. She says it’s hard to rebuild anything new and it will take her some time to feel differently. She breaks up again and he ignores it and keeps acting as if they’re still together. And so they keep going. At some point she says she really broke up this time and actually started dating some other guys. I was happy for her that she finally managed to cut it off. But after a month she started talking about her ex again: that he still has the keys to her house and sometimes comes inside her house when she's at work to 'check if there's still pictures of them hanging on the wall’ and he would leave notes for her. That he brought her flowers and gave her a ride to the airport which was “actually handy”. That his mother is very ill and she feels obliged to ask him about it. Because that she can't be rude to him after “all he did for her”. That she feels afraid to slip back into the relationship again, because like always he just keeps acting like they are still together. That he asked her if he could stay at her place for one month because of a new job close to her house, and that she feels like she can't refuse it because he still pays part of her rent. She also sent me a picture showing a big cut in her nose. Saying it was an “accident at the gym”. (?) No surprise, a few weeks later she announces she’s back with him, saying she wants to give another chance to the person that has been there for her the past 5 years in all the good and bad times. My question: Is this just two people who just can’t be without each other and are addicted to breaking up and making up? Or is this a slowly developing (emotionally) abusive relationship? Doe she really love this guy and is she making up his “manipulation” as an excuse to go back to him? Or doesn’t she even realise his behaviour looks pretty toxic from outside? I know she has been in a physically abusive relationship before, has abandonment issues, low self esteem etc. Where do you start drawing the line between: “if she really wanted to break up she would have done so ling time ago” or “if she really wanted this guy out of her life she could change her locks and call the police” and “she’s being emotionally abused and I as a friend have to take some kind of action?” And if so, what could I do from a distance? What do you guys make of this? And what’s his and her role or motives and behaviours? Could they be abusing each other? Could she be abusing him? I find it so difficult to tell. Hope to find some answers from people who know about this stuff.