a Jew in a "marriage" with a non-Jew is in serious violation of Torah law. the non-Jew, often when this is explained to them in a clear, non-offensive way realizes the danger their spouse is in and comes to a reality that they themselves can make a difference. one must already have a sensitivity to religion in general to make that leap. but i have seen it. this same logic breaks up people when dating as well but should only be used by a competent Orthodox Rabbi who can explain the situation as to the children, afterlife, the mitzvahs, etc...
but once in the marriage, efforts should always be made.