In short you are responsible, but not until the landlord actuall does and pays for the repairs.
I am not sure this is accurate, dear TC and CF (peace to you, both!). First, the landlord's insurance should be used... unless the damage is less than the deductible... or the landlord isn't filing a claim for some reason. If he does use insurance, the insurance company has a right to come after you. If he doesn't, he may have a right to ask you to pay, based on estimates (versus him paying and you reimbursing him). She MUST, however, give you copies of the estimates - she can't just give you a number. Even if she does the work herself, she has to itemize what the labor and materials costs will be, and include some kind of proof.
The issue is... pay by WHEN... and that information should be included in your lease ("Tenant agrees to pay/reimburse landlord within ____ days"). If not, then she can give you what is called a "Demand Letter"... which usually has a 10-day performance period... or a "Perform or Quit" Notice, which has 3-, 10-, or 30-day performance periods. Again, this can depend on what your lease says.
It doesn't matter "how" the fire occurred... unless it's possible you/your gf WEREN'T at fault (i.e., faulty stove, electrical, etc.), in which case you should consult an attorney. If you are certain it WAS your/her fault... then it would behoove you to do all you can to work with your Landlord to resolve the issue. Why? Because you could work up a "bad" reference in the event you have to move. Note, if you have to move out while repairs are being done, you are not only responsible for the temporary digs... but that unit, as well. Yep... you have to pay rent on both!
Just so you know... she most probably can evict you, regardless: you have an obligation not to damage the premises and it doesn't sound like you carried insurance to cover if/when you did. So it really would be up to her. If you paid her as she demands, a court MIGHT be reluctant to allow an eviction... on the grounds that you performed per your agreement with her.
An attorney can be expensive and hiring one just to tell you, "Well, you need to pay her," may not be prudent; however, I would never tell you NOT to consult an attorney. You absolutely SHOULD... if you can't find a way to resolve this without doing so. Another alternative, however, might be mediation. Check with your local Bar Association and see if that might be a less expensive way to go.
But your BEST move would be to TALK with her... and try to work a payment arrangement out.
I hope this helps... and hope all works out well for you, dear one!
Peace to you!
SA, on her own...