From being on of those kids that as stuck in one of those battles you are talking about I can say a couple of things from experience.
1. If you are in a small town area go outside the area for a lawyer. The local ones are all in each others pockets and crooked like the local priests but that is another topic for a different day.
2. Lots of times an absent parent that up and decides one day to be an actual parent is just trying to throw their weight around. Once the actual thought of making a room up, feeding, clothing, and all the other stuff they have not been doing or paying for that comes with it comes to light their tune changes real quick.
3. The Mother in this case should first write out all that this father has missed (birthdays, christmas(if they do this holiday) etc..) Also just what has this loser contributed to her sons life.
4. Fore most you need to tell her to GROW SOME BALLS and not to back down from this clown and take the fight to him. If her son needs help and she is willing to get some for him that is the main focus here and the most important.
5. The most important is focus on the child. It is very easy for adults to lose sight that they are fighting over a part of them not a thing. I know it would have been nice when I was a kid watching all the "grown ups" fighting over what was best for me if they once stop and thought just what that was.