Consult your contract, look the law up and find the exact statements.
If it says she has the right to break any contract due to her going to active duty, that doesn't imply that she doesn't have to pay rent, it implies that her name is now null from the existing contract. However, if the law implies that she merely isn't required to pay rent, then I would think that it would mean you aren't responsible for picking up the slack.
In any case, you do have to consider that you now have a two person apartment with one person living in it. Just because their is one person, doesn't mean you have less of an apartment.
To summarize, don't assume anything about the laws, consult a lawyer if you need an interpretation (it may cost a little bit, but you'll have some clarification.
HAVE A BACKUP PLAN. What will you do if you do have to pay the rent? Can you start looking for a new roomate? You may be able to tag yourself onto that law, depending on the actual text. If the contract was signed jointly, then I would think that the only way it can be null is if it's entirely void, not just removing a person from it. So you may want to find a place you can chill for a bit if that does happen.
I'm not a lawyer, nor do I even know the exact text of the law, but I would think that in a situation like this the better side of people may show through and your landlord would help you out in some way.