Fathers custody

I live in a state where men get screwed over on getting an sort of custody of their children. I have seen drug addicted mothers get full custody over a clean and sober dad.  
Well a friend of mine recently went through a divorce. His ex and him split custody 50/50. It was working great for them! No problems. Well today he received a notice that his ex is filling for full custody. And she WILL get full custody 100%.  
What's sad is that this guy is amazing with his little girls! He loves them more than life itself. He was even a stay at home dad for the past 5 years. But he is now losing his kids because the mom can. 
Does anyone find this fair? What about the kids? Why do mothers do this? I understand doing it if the dad was a dead beat... But he isn't. I see this a lot. I need some help understanding the moms position on all this.