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Thanks for writing. I am sorry to hear about your situation.
I think that according to Scripture she is already "one flesh" with this man. I Corinthians 6:16 says,
"What? know ye not that he which is joined to an harlot is one body? for two, saith he, shall be one flesh."
So, apparently you don't have to be married to be considered "one flesh".
But, being "one flesh" does not mean that you automatically become "married" in God's eyes either. When Jesus dealt with the woman at the well He said to her,
"For thou hast had five husbands; and he whom thou now hast is not thy husband..." (John 4:18).
She was living with a man, and thus "one flesh" with him, but Jesus did not considered her to be married to him.
I hope this answers your question. Once again, I am sorry to hear about your situation, and pray that God would give you direction and encouragement while you deal with it.