It would be a big mistake to focus on trying to get the husband to leave her.
What if she is meant to convert? Kiruv is a slow process. Wait for him to become more observant, and once he does, and she is exposed to it, perhaps she will desire to become frum and convert halachically. If not, their religious differences may well lead to divorce. Let him make the decision on his own. Even if it takes her a decade to convert (or for them to amicably divorce and re-marry appropriate parners), that will show the wait was worth it.
People have to make decisions on their own time. Lives are at stake. One has to be wise and know what battles to pick, when to push an issue and when not to. Why focus on one mitzvah that would cause the most suffering when there are 612 more?
Another thing to consider is that many who are mekareved do not stay with Yiddishkeit long, and slide back until their own secular ways (or even convert so some other religion, c”v’s.) Is it a good idea to whisk a starry-eyed BT away from his family, plunging the family into despair, when he may well be no more religious than before within a few months, having destroyed his children’s lives for nothing? Often the slower, more gradual process is more effective.