The reason I don’t think this would be a good idea is because of the consequences it would have. Imagine a lady coming home one day and telling her agnostic husband of 15 years whom she has a good relationship with, and not to mention 3 children with, that they are actually NOT married and that she is actually married to her first husband still. And for that reason they must now stop living as husband and wife and start living as brother and sister… complete with separate bedrooms and all. The man wouldn’t understand, and would probably and rightfully feel very betrayed, confused, and not to mention devastated.
Most men in this scenario would legally “divorce” this woman, and be free to date/marry someone else… and he’d have every right to do so. Keep in mind also that this would not only effect them and break them, but it would effect the children as well. What a mess that would be! Complete with divorce and all!
Unless the man is by some miracle ok with living with his “wife” as a brother and being celibate, I think the best thing for this lady to do would be to continue to attend mass but not take the next step to officially become Catholic.