You can talk to her about why she loves him , what she wants form life and marriage and just engage without moral judgements.
It would not be responsible for him to marry if he could not find a job, how would they live, starting out on one salary nowadays seems impossible. You don't need to tell your dd that but you can help her join the dots.
I am 'religious' (Christian) I did sleep with someone before marriage, but not my dh! Long story, I had a time of not being so religious.
Our marriage is fine and it all worked out but we were 30 something not 20 something and I've told my own dd what she does re sex before marriage is her own choice.
Living together doesn't guarantee that you will only marry the right one. Lots of marriages fail and the majority of those people have lived or at least slept together before marriage.
May I ask what kind of church (domination) they go to, please? We are C of E which tends to be a bit more liberal minded in senses than some other denominations.
Where they both 'religious' before they met each other?
Last question, do they equally feel the same for each other?
All the best, stay calm, don't alienate her (or him); even if they marry and divorce later, it's not the end of the world,. Most of all, as you know, it's not your choice. But you are right (IMHO) to be at least a little concerned.