It would be the same as a baptised person marrying someone not baptised. It actually happens a lot these days. The elders will tell them all the scriptures about only marriage in the lord and all that.
if they are good elders and they can see this person is so lonely and hasn’t been able to find anyone in the religion. If they are really in love and would be so much happier getting married to this person then I hope good elders would see the situation for what it is.
the old school bad elders would maybe try to threaten with marking or restricting answering up and things like that. But they now know those things don’t have any effect at all.