Is it permissible for me to marry a girl who was emotionally involved with a friend of mine, and what is the interpretation of my dream of having intercourse with her in a desolate place resembling a mosque?

The correct approach to marriage is to seek the girl's hand from her parents. Her emotional past does not preclude this. Rather, it is forbidden to propose to a woman if she has already accepted a suitor and is content with him, based on the Prophet's (peace be upon him) saying: "A man must not propose to a woman already proposed to by his brother, nor bid over his brother's bid." This applies if they have drawn close and become content. There is no impediment to proposing to the woman mentioned. As for dreams, no legal rulings are built upon them, nor are they relied upon to determine future choices. Instead, reliance should be placed on seeking Allah's guidance (Istikhara) and consulting people of wisdom and good counsel.