This thread has been moved to an appropriate forum...
As per this advice:
http://forums.windowscentral.com/ask-question/292973-who-should-post-ask-question-forum.html WC members are free to post in all areas of the board. Please use Ask a Question only for time critical help issues that affect the immediate functionality of your device.
Apps that "lock" other Apps in Windows Phone are a placebo. The Apps are still available to open via the All Apps list. Security sandboxing within the OS prevents one 3rd party App from being able to exert that level of control over any other Apps.
Locking your phone is still the best security. If you let someone borrow your phone and they can't stop snooping, you need to reconsider allowing them access to your device. This is not acceptable behavior from a "friend."