How can you be in a relationship with her for 4 months and still be a virgin? Are you actually WITH her or are you sanctioned to each other, with no sex until afterwards? Does her being "tainted" reduce her social status/ caste below yours? What would happen to her if you betrayed her trust?

Are you afraid that she will think of him as a better lover than you? Do you think that she will go back to him? Does her admission diminish her in your eyes?

Surely, in the last four years of being friends, you must have known she had a boyfriend? Was expecting her to be a virgin realistic?

Her life. Her choices. Her body.

If you can't accept this, perhaps you should go your separate ways. You can find a virgin and she can find a man strong enough to love her for the person she is.