When my nomo husband does go to church we discuss the talks. This guy was orbiting so strongly that he changed his religion before they were even dating. It's gonna hurt, and it's gonna break her heart, but much less so than waiting until you've both committed to each other and start discussing your future together. And Mormons were basically encouraged to marry other mormons. The importance of tithing your money. She is going to be taught for the rest of her life in the church the importance of missionary work and eternal families, and Priesthood in the home. While dating is a good time to get to know someone, if you are young it is better to go on group dates. Divorce would have wiped him out and he would not have been able to ever stop working. She still lives in her hometown though. Religious differences, however are real.
I do wonder if you ask this blog just to get supporting advice. I have been understanding of the demands of his career for many years, but I have come to realized that I'm not happy living this way. Ignore the busy-bodies who want to condemn your significant other. It's tough to date a med student let alone a resident. When DH comes in he is still gone more than half the time. I have to breathe. I've never understood that bit of the LDS culture. The church can be a very cruel place for single people. She may never join the join the church.
I'm glad you both have found a way to get past the incredible forces that are working against you. No where did I say, nor I think indicate, that I thought this wasn't a complex issue, or that this girl is a caricature. He would be leaving his career, something he has worked so hard for and his passion for his family.
If you can't do that, realizing that your partner may never come around to your side of things, you are not ready to marry this person. If she doesn't care that you aren't a member now, if your relationship goes on long enough, she is going to care eventually. That's the real issue.