Is it valid to fast and pray if a man and a woman live in the same house while intending to marry?

It is not permissible for a man to live in the same house with a woman who is not his mahram (definition), due to the forbidden seclusion (khalwa (definition)) involved. If, in addition to that, a heinous act (fahisha) is committed, the matter becomes even more reprehensible. They both must repent, seek forgiveness, and vacate the residence immediately. The intention to marry does not make this permissible. However, it is permissible to marry her after her repentance. As for the prayer and fasting, we hope they are valid.