First of all, I would have a long talk with this guy on how this woman treats his daughter. While he's out screwing around, the wife has to know something/suspect and may be taking it out on the little girl. That is not right AT ALL! Also, if he's not going to protect his daughter, maybe someone of higher authority will (like his first shirt if he's Airforce). I knew of a little girl that was killed at the hands of her stepmonster and she was only 3. You don't know the full extent of it since you're not in their home and it can't get any better but will only get worse.
As far as the cheating, it's not your concern. I would say different if she was a friend but seriously, I'm more worried about this little girl and the long term damage she will suffer from this monster. Protect the innocent...protect the children. Forget the adults, they made their own beds...now they can lie in it!!!!!