My guess is that this person's suspicions are well-founded, and that she knows it, and has known it for a long time.
As a longtime family law attorney, I am shocked, disgusted and sad, about the frequency with which I hear this scenario, and action-oriented in my recommendations about dealing with it. It is best - without judgment, accusation, or threat - to ask one's daughter if her father is relating with her in a way that makes her feel guilty, uncomfortable, pressured or "special". The problem is that this man - bolstered by the fact that this is not his biological daughter, and that that fact makes his behavior less wrong - has very likely been courting this girl for years. Little by little he has coaxed or disciplined her away from social interaction with peers; dates are out, his word is law, and,, worst of all, he has systematically disrupted the relationship between the mother and the child. These men tell the child that the mother would not understand, would be jealous, doesn't really have the child's best interests at heart, in fact, doesn't know or understand her as he does.
If nothing can be determined quickly about the truth or falsity of the mother's suspicions, it is literally life and death for her to err on the side of caution and protection of her child. Have the man leave or be removed from the home. Permit communication only when mother is in earshot or on the extension. Because 18 year old girls always know better than their mother - intensified when a father is actively undermining - if not destroying - the relationship between the girl and her mother, outside help may be required. If the 18 year old balks, find a way to work professional help into the picture. The father cannot return until all suspicion has been laid aside.
Sometimes the mother's response is affected by her own history of sexual abuse, and a part of her considers the abuse of her child inevitable. Often there are serious problems in the marriage. In a very recent case, the husband is impotent, and the couple have not been intimate for years. As the older of her two daughter said: "dad is trying to make us into someone he'd want for a girlfriend or wife." This lovely woman could not see how her husband's excessive attention to the girls did not make him a "wonderful father," but a sexual abuser.
