have you asked her how she would teach the kids? would this guy agree to raising the children in the faith, which is part of getting married in the Church? They might not be thinking ahead very much, and need to realize that having one parent going to Church and the other saying he never will is damaging to the kids. Lots of times, the kids wind up not believing in anything because that of course is the easiest option.
Sometimes it works though: when we were little, my mom took us to Mass and we noticed Dad wasn’t going. We asked him why he didn’t go to Mass and I guess he needed an answer for us or something (we were really small). Anyway, he started looking into the faith more and saw how much sense it makes! My dad is a really logical, rational sort of guy. He converted to Catholicism when I was about seven.
I could tell you similar stories about many of my aunts and uncles, the living together first part included.
You know why I think it worked out okay in my family?
Because my grandmama goes to Mass everyday and prays the rosary everyday, for all of our family’s intentions.
I would say, don’t badger them: state your case once and very clearly. Then pray for them. When the time comes that they need help, they’ll be more likely to come to you because you won’t have put walls up between you.