Take her into your house.
There is something "missing" inside of her that she feels that she needs this "man." Her "father" sounds like he's been emotionally away from his daughter, and this is fueling her need to be with this "man." Keep her busy, both physically & mentally. Get her out & about - whether that's going hiking, beach, boating, school, etc. At 19, there are plenty of other men who will find her worthwhile, sexy, fun, ...and they will not be married, will not lie to her (well, at least not as badly), and not have 3 kids. This "man" isn't going to leave his wife, too much alimony, child support, loss of social status in his family, etc. This "man" just wanted a fling.