HELP

I need help in helping a friend, He is a father of twin boys that he has had custody of for 7.5 years. The mother ended up leaving and not returning for that period of time...She gave full custody of the twins to the father...now she has come back after 7 years and is trying to gain joint custody of the boys. She pays no child support nor has she sent monies to the father for christmas, birthdays, or medicial. She left the children in order to get help with her drug use. She says she has been clean for 5 years but in that 5 years she has not seen the twins but 2 times. They are headed back to court for the order that she put in to gain joint custody. He didn't file for custody on the terms which was 7 years ago that she would get help and return to help with the twins which she has not. I am wondering if he can file for back child support even though there was no order and if not, can he maintain his full custody rights that a judge ordered him to have? The help would be grateful thanks..