Well it all depends on her type as I said before, if she has type1 genital then the chance for you to get it down there is close to zero as you already have it orally (you get cold sores, right?) and as a result you have anti bodies against type1, so you won't get re-infected but if she has type 2 and you are negative for it then YES it is possible for you to get infected but it doesn't mean that you are infected for sure. That's why I mentioned that it's important to know the type!
And type1 and type 2 never convert to each other, if you are exposed to either type then you'll get that specific type. So my 2 cents is: If she has type1 then you don't really have much to worry about but still you guys HAVE to avoid sex during outbreaks for sure and if she has type2 then you need to be tested to see if you have it (a lot of people have it with no symptoms) and if you don't have it then just avoid sex during her outbreaks, try to use condoms and only this reduces the transmission rate to 4% per year and if she considers going on medications daily that breaks the chance to half again.