French-speakers use fermé for both meanings and usually the context can help you to determine whether they want to say closed or locked.
Funnily enough, the word ouvert (open) is subject to the same ambiguity (open / unlocked).
French-speakers do not often use the word verrouillé (locked) and most of the time refer to it by fermé à clé. Also they often refer to closed but not locked simply by ouvert.
If the context is too ambiguous to be sure what meaning has the word fermé, then just ask if the person is talking about fermé à clé (locked) or just fermé (closed).
Usually, when a french-speaker says La porte est fermée, he or she means by it The door is locked. This is a shortcut for the regular expression la porte est fermée à clé. [by the way, for those wondering, "clé" is another correct spelling for "clef" which, as a native speaker, I do prefer.]
As a result this can be a little ambiguous but usually the context tells you whether the person is speaking about closed or locked.
As-tu fermé la porte en partant ?
Did you lock the door when leaving?
Here it seems quite obvious the person is talking about locking the door, because you rarely forget to close the door when leaving but could possibly forget to lock it.
- Comment es-tu entré ?
- La porte n'était pas fermée.
- How did you get in?
-The door was not locked.
In this case again you can understand the second character is speaking about the door being locked. If the door was supposed to be closed but not locked, the first person wouldn't have asked how the second managed to enter in the first place.
Quand ma porte est fermée, c'est que je ne souhaite pas être dérangé !
When my door is closed, it means it don't want anyone to disturb me!
There, the person informs somebody that when he or she doesn't want to be annoyed by people entering the room, he or she closes his or her door so people know they are not welcome to enter. He or she wouldn't have had to say this if it was about a door being locked - in which case people simply couldn't enter.
In the case of your girlfriend, if the door was not closed, then she would have nuanced her affirmation in a more explicit and somehow "dramatic" way : she would have said "The door was open!".
That being said, the word ouvert, "open", is as ambiguous as its antonym. We often use it to indicate a door is closed but unlocked.
- Pourrais-tu aller chercher du papier dans mon bureau ?
- N'est-il pas fermé ?, demanda-t-il.
- Non, répondit-elle, il est ouvert.
- Could you please go and get some paper in my office?
- Isn't it locked?, he asked.
- No, she answered, it is unlocked.
Here you can see the double meaning of both the word fermé and the word ouvert.
The masculine character doesn't need to know of the door is closed, it wouldn't be useful to ask. However he needs to know if it is locked because if it is, he just can't do what the feminine character is asking him. Hearing the question, the feminine character understands that because it seems logic, so she answers with the word ouvert to tell him her office is unlocked.
However despite the context helping most of the time to find out if we are talking about locked or closed, all cases are not obvious enough . This is why it is not rare nor strange to ask:
- La porte n'est pas fermée.
- Tu veux dire fermée à clé ?
- The door isn't closed.
- You mean locked?
(well this isn't exactly translatable but you get the idea)
If you are wondering what the other person is meaning, just ask. As simple as that ;)