AppLock is a popular locking Android app. One can use it lock certain areas on ones phone. You can as well know how to bypass AppLock lock pattern without contacting the owner for a lock PIN.
This will work for you in a condition. If the person locking his/her apps is not too smart to lock Settings. From the name, its main function locking up apps from sharing contents from disturbing friends.
This will not work if the locker tries to lock the settings, then proceed. Root permission is not required.