Is it permissible, in Islamic law, to help a friend with printing work without her parents' knowledge or approval, and without taking payment?

Obedience to parents is obligatory, except in disobedience [to Allah]. Helping a friend is commendable. If the obligatory and the commendable conflict, the obligatory takes precedence. Therefore, obedience to your parents is obligatory if they do not approve of you going to the printing center, and disobeying them is forbidden. Try to persuade them in the best manner, but if they are not convinced, do not disobey their command.