Help divorce/child support

Cydney

A friend of mine is getting served divorce papers, she filed that they have been legally sperated since December 2016. He is in prison momentarily. She filed to have him pay $195 for all three children upon his release. Could have have to pay backpag child support? And when is the soonest he could file a motion to modify the custody arrangement? Also does it matter as of sole leagal custody and sole physical custody? This is in Idaho.