Or you have unsecured network storage on your network. Anyone on the same wifi will have access to all devices and if they are unsecured, they can get anything.
May be ask the "Friend", if she is really friend, she will help you secure the network properly.
Either way, may be change wifi password is also good idea.
Next time, create guest wifi network and isolate that from your - most reasonable routers can do this. You can then provide wifi to guests without exposing your own data to them.