It could mean both. For clarification, one may ask: "Deine Freundin oder eine Freundin?", e.g. if someone ist telling about something that happened between him and the friend, and the story would have a completely different meaning according to what kind of relationship they have. In such a case it would not be considered nosy to ask, either.
"The Girlfriend" is a bit unusual in English. Usually it's with a possessive like my/ his/her/your I think this is because it defines a relationship between people.
I think saying "the girlfriend" depersonalises the relationship and reduces it to a role, title or object. Maybe thats what the German sentence means to do?
'die Freunden' doesn't work, because Freunden is the dative plural but die is the nominative/accusative plural. As a result you'd never hear this combination.
Die Freunde essen zusammen = "The friends eat together'
Ich kenne die Freunde = "I know the friends"
Wir spielen mit den Freunden = "We play with the friends"
As you can see in the last example, in dative case the plural definite article is den. Dative case is required here because of the preposition mit, which always triggers dative case in what follows it.