Well, first of all JWs are only allowed to 'date' within their faith and even then its with a view to marriage. So what she has done - dating a 'worldly' person - that is one who is not a JW - is a big No No. She can get into trouble for this but perhaps not disfellowshiped (if things haven't gone too far) - that is being excommunicated which is not the same as disassociation where one chooses to leave by their own accord.
However, if she has been dancing the 'tango' with you in the bedroom, or petting the one-eyed snake she is in a heap of trouble and can be disfellowshipped. This can be a very traumatic experience for her and her family. If one's daughter or son is disfellowshipped and continues to practice 'wrong doing', JW parents may ask her or him to leave the household. Particularly if there are siblings since they are now 'bad apples'. She could essentially be cut-off from her family, JW friends and left to fend for herself. Its a frightening prospect especially if you are a 'born-in' and all your friends and family are JW. It has driven some to suicide!
So the question is, does she want to choose JW over you? if thats the case, she will break the relationship, confess to the 'elders' and perhaps get disfellowshipped anyway. But she will continue to attend meetings in silence and accept her 'reproof'. After perhaps a year or so, she will get 're-instated' and will have her family and friends back. This will be the end of the line for your relationship so both of you may decide to have an amicable break-up.
If she chooses you over the JW religion, then she may 'disassociate' herself which is a clean-break. Her family and friends won't talk to her anymore but she will have you. Or she can try 'fading' - this is when one slowly disassociates from every day JW activity such as meetings, service but still is a JW in the books. But if found out for wrong doing, said person can be disfellowshipped. This is possible but requires careful planning and moving away to an area where no body knows you for a fresh start. Risky maneuver especially if she wants to keep in touch with her family.
How much do you love her? Is marriage perhaps on the pipe-line? There is a way for her to have her cake and eat it too. However, she will need to come clean with the elders and get disfellowshipped. She goes through her 'reproof' and you continue your relationship and get married to her during this period. She then gets 're-instated' as a JW and gets to associate with her family and friends. They will associate with you since you are both married and as an 'unbelieving' mate and you will get love bombed by the congregation in the hope you get converted. Just keep it sweet, attend the once a year memorial, the sunday of the annual JW convention and things will be cushti.