Is the marriage of a Christian woman to a Muslim man Islamically valid if the marriage contract was concluded in the presence of witnesses, a dowry, and offer and acceptance, but the guardian was the father of her Muslim friend instead of her original guardian or the judge?

The one who undertakes the marriage of a Christian or Jewish woman (Kitābīyah) is her guardian from among the people of her religion, or the Muslim ruler, or from among the people of her religion if no guardian is available. It is not permissible for an ordinary Muslim man to undertake her marriage.

Therefore, if your girlfriend's father undertook her marriage, the contract is invalid and must be annulled. If you both wish to continue, the contract must be renewed in the correct manner.

The children resulting from this marriage are attributed to their father, as a فاسد (invalid but not void) marriage, after consummation, entails the effects of a valid marriage.