Welcome to Dream Views Azra.

If true, this is really an interesting story, as one's body is normally paralysed while dreaming, except for the eyes. It is proven that dreamers can gather information with their five senses while in a dream, but they can't move their real body.
The only explanation I have is that your girlfriend was in a kind of half sleep/dream state due to her tiredness. I never heard that such a state can be archived, where you can talk with the outside world during a dream neither that something like this excistes.

But in fact, I'm no professional, neither am I a scientist.

Interesting story and good luck on future lucid dreams

Cheers
Thorim