If God is real, then who is to say that he/she/it/they can't talk to people?
If you don't believe in God, then obviously you wouldn't believe in God talking to people.
If you say you believe in God, or Allah, or whichever, I think most religions believe that their Deity is capable of communication.
On the topic of your friend; a lot of people claim to hear "voices". If she loses the ability to tell right from wrong, or this "voice" starts telling her to do things that are way outside her normal values or beliefs then she should get help ASAP. If the "voice" is turning her into a better person, maybe it is God?