Praise be to Allah.
It is regrettable indeed that anyone would fail to adhere to the religion which Allaah has commanded us to adhere to. What is even more distressing is the fact that a Muslim woman would have a relationship (of any kind) with a non-related Christian man for ten years, when she knows that this is something which angers Allaah, Who has forbidden believing women to do such things in the Quran (interpretation of the meaning): they should be chaste, not adulterous, nor taking boyfriends [al-Nisaa 4:25]. Equally upsetting is the fact that what this Muslim girls father fears most with regard to the marriage described is not Allaah, but what his mother will say and how his reputation among his relatives will be affected.
In any case, there is no way out of this problem now except one: this man must become Muslim in a real sense, and start to practise Islam, and he and this girl should repent for what they have done. Then marrying him will be permissible, and what people may say does not matter, so long as everything is done in accordance with Islam and in a way that will not earn the anger of Allaah; the matter can also be explained to close relatives. If this is not possible, then the relationship with this man should be cut completely, and all thought of him should be dismissed from ones mind. If, my sister, you feel that there is some harshness in this answer, then by Allaah it is only in your best interests and out of concern for you. We ask Allaah to accept our repentance and have mercy on us, for He is the One Who accepts repentance and is All-Merciful.