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?