concernCitizen:
Disfellowshipping (DF) and Disassociation (DA) are similar in terms of the consequences (she will be cut of by anyone she knows and considered death until she is reinstated, a long process). They differ in that you get disfellowshipped if 1) she is baptized and 2) you are found guilty of a serious sin by a judicial committee (a tribunal of 3 elders), for instance if they learn that you have had sex or just slept under the same roof.
Speaking of which, is she baptized? (this is very important)
DAing is most commonly something she does by writing a letter and asking for it. Otherwise she can be "forcefully" DAd if it for instance is found out she is a member of another religion. Insofar as you are concerned, DF/DA are the same.
The elders have a manual that lay out all these procedures in detail which is secret. She can't know that you are reading it but you can find it online (google: shepherd the flock of God).
If she is baptized, the best thing that can happen from your perspective is that she fade out. You fade out by slowly stopping to go to the meetings, don't do anything "wrong" (like living with you before you are married) and possibly feigning a depression. After a few years they will begin to leave you alone as long as you don't rock the boat. Then she will be said to be "inactive" and you two will get more breathing space. The bad news is that she will likely loose all her friends who are JW and be viewed by her family as someone who couldn't hack the spiritual life and she will constantly reminded (because the JWs are constantly reminded to do so!) that she should "return to Jehovah".
If she is not baptized you two are lucky :-). She can't be DFd or DA. If she choose to be with you it will still disappoint her family a lot (you can expect that you will be a disappointment to her family), but it likely won't have so severe consequences. You can still expect her friends not to stick around since she is obviously not "good association", and my guess would be that if it is found out she is sleeping/living with you it will strain her relationship with her parents a lot.