Luge, there is no advice you can give her because she committed adultery. As a JW, she is not scripturally available to re-marry even if a divorce does happen.
But let's say the divorce happens and she goes against Botchtower and you 2 are together.
First off, you do realize, that JWs HATE Catholics, right? Second, she is going to live in guilt because her marriage didn't work out and she committed adultery. So she has a ton of crap to deal with. You're going to need to get her some professional counselling. Also, the JWs may hound her and will likely look to disfellowshipping her which means her family will then have to shun her.
You are in for one hell of a nasty, rough ride.