From all I've read on the subject, doing it once is not the problem but when your child does it and his/her child does it, generation after generation, they run into some real problems. The pagan muslims have been doing it for hundreds of years. This guy says "three generations", more likely more than a hundred generations.
The flap a couple of years ago about that mormon cult in Texas, (The real mormons), where the men were marrying child brides also uncovered the fact that the cultists had been intermarrying for generations, as a result a lot of newborns died and many that lived were severely mentally and physically disabled.
Before the law was handed to Moses it was fairly common practice, Abraham and Sarah were half brother/sister.
If this guy is Muslim he may have just wasted his time and money. If she has sex before marriage he will just have to kill her later.