How to ask a female friend to stop harassing my boyfriend?

Question

Back Story:
When we were in high-school, a male friend of mine (let's say X) developed a crush on another friend (Y). Ever since he told her about this, she started acting as if she owned him. She refused his propositions in very clear terms, but gave a shout out to the rest of us that he was her best friend, and that he would do anything for her. To further prove her points, she got X to handle a school bully, which resulted him suffering mentally and sink into depression.
She, on the other hand, had several relationships over the past years, which barely lasted a couple of months. Even then she made sure she kept calling X her best friend, although now they were in different colleges.

During the 4 years in college, I got close to X and we both realized that we actually had a lot in common and enjoyed each other's company. Eventually we started dating. Now, when this came out, Y suddenly had some weird inspiration and she told X that she has begun to like him, and hinted about wanting to be romantic with him. He refused but this triggered his depression again. (He was fine for at least a couple of years.)

Me and X took a 7 month long break due to his depressed state. During these 7 months, he didn't talk to anybody and kept to himself. And once he felt better and we got back together, Y was back demanding a romantic relationship!!!

I fear that her company is harmful for his mental health, and her constantly wanting a romantic relationship might get him depressed again. Where I live, mental health is still considered a taboo. So I cannot go around telling people what my boyfriend goes through. Only his siblings and I know about this. He will be starting therapy about 7-8 months from now.

How can I tell Y to stop bothering my boyfriend, and stop this charade or whatever she is doing?

Answer 1

Being in somewhat similar position and knowing few people who have been in chokehold.

Only specific, decisive action will.

Your boyfriend need to tell Y

That ship has sailed, the train have left the station, the goose is cooked. The things have changed in the last 6 years and I'm no longer interested.

Y don't care about your boyfriend or yours comfort or state of mind. She cares what she can get from the relationship. You also need to remove any consideration of how she she "might feel". That also include not thinking what she might tell to your common friends. You don't have to tell anything about X mental.
Y is hitting on your boyfriends. Whatever history they might have (especially if it was one way, not-repricocated) don't give her any right to act that way and it's not only in bad taste but create unnecessary tensions.

1. It should be your boyfriend who tell her that he don't want anything. Without saying sorry, without trying to cushion the "blow". Accepting that this might cost some friends. He should not take care for others. He should think about himself.
2. If, after the initial refusal, Y will be persistent then you might act as a "deflector". Harsh talk, public outing (as in informing common friends about her doing and informing you won't attend meeting where all of you are present)

You don't have to mention, or treat this, mental issues. As I wrote before, trying to snatch someone else partner is, in most cultures, a big No-No.

Answer 2

It's harder to fully answer without knowing your boyfriend's position & stance on this matter. The relapse of depression clearly shows that she still holds a grasp on him be it romantically, psychologically, or both. You and your boyfriend have to work together and find out Y's foothold on him.

"How can I tell Y to stop bothering my boyfriend, and stop this charade or whatever she is doing?"

Asking Y to back down is viable, but it should not be the only action you take. Try to work/talk things out with your boyfriend and draw a clear line on this matter, so that you both could move forward. Y is clearly toxic and manipulative based on what you described, so show him the light.