Can grown unmarried men or women keep Mustahab fasts without their parents' permission? In the case that their parents don't know and they are told to eat, can they break their fast?
Recommended fast can be broken any time. It is recommended to eat or drink if a Mo'min invite you while your fasting a recommended fast with out telling him that you are fasting.
No need for permission of parents in recommended fast nor in other recommended worship.
Wassalam.