Concerned for my best friend's safety

Over the past few months, my bestie has had a friend who's become toxic and obsessed with her.

It started normal enough, but gradually after a while he started complimenting her numerous times a day. After a while she began telling me how uncomfortable she was feeling and she looked genuinely concerned that her boundaries were either getting pushed aside, or as he claimed 'he couldn't see them'. I've seen the messages and she's just tried to be a good friend listening to his many problems.

Soon after he confessed he loved her and wanted to be in a relationship. She didn't take this very well and firmly stated they are friends and only friends.

He doesn't have many friends or supportive family.

He's got significant MH issues, and has a long history of falling in love with women who are nice enough to be friends with him but they aren't seeking anything more. It seems to me that anybody willing to be there for him, he latches onto and clings to them for dear life.

His behaviour began to escalate. He messaged more and more, became demanding, manipulative, he guilt tripped her many times and began showing signs of jealousy if she was out and about with friends and what not. He also began speaking to her as though she was his girlfriend and always asking where she was.

She's definitely tried to maintain her safety.

She's been seeing a man for a while now and her 'friend' keeps comparing himself to this great likeness of her partner before he 'lost himself'.

He keeps reiterating his feelings for her but she's remained firm on that subject.

Recently she found out he'd been posting about her online extensively as though they were in a relationship together.

It's always the same story, always has different excuses for his feelings, they don't want a relationship, usually he's ghosted but she's scared that would make things so much worse.

She confronted him, at first he denied it all then came clean after she sent him the screenshots.

Then a close family member of his reached out concerned about her safety and well being as he is well known for stalking the women he becomes infatuated with.

What I'd like to know is how best to support her going through all this? How can I help ensure her safety?

She's distanced herself for the moment but every time he manipulates her back or has another 'crisis' he can't cope with. He seems to desperately cling to others in very unhealthy ways to cope with life.

Over the weekend she has become engaged to her partner. He's also fully aware of this situation and also wants to keep her safe obviously.

I'm concerned this should be a wonderful and joyful occasion but many of us can see she's worried if there will be any issues, almost like she's having to look over her shoulder.

She is surrounded by a loving family and close friends all aware of what has happened over the past few months and we all just want to see her happy.

I know I'm fearing the worst, but he doesn't live far from her and based on what his own family have said reaching out to her, I'm really concerned.

Any advice is much appreciated!