Your friend’s theory is known as polygenism, and it was addressed by Pope Pius XII in his encyclical Humani Generis:
The faithful cannot embrace that opinion that maintains that either after Adam there existed on this earth true men who did not take their origin through natural generation from him as from the first parent of all or that Adam represents a certain number of first parents. Now it is no way apparent how such an opinion can be reconciled with that which the sources of revealed truth and the documents of the teaching authority of the Church propose with regard to original sin, which proceeds from a sin actually committed by an individual Adam and which, through generation, is passed on to all and is in everyone as his own. (HG 37)
Where did Cain’s wife come? Although their names are not recorded in Scripture, Adam and Eve had other children, including daughters—Cain seems to have married his sister. This was necessary (for a time) to propagate the human race. St. Augustine explained this in The City of God, book XV. When the necessity for sibling marriage ended so, too, did God’s allowance of it.