I honestly don't see the problem at all except him not working yet. Hopefully he will find something soon. I'm not religious at all but if that's who she is then that's who she is. I actually think the pre marriage counselling people have described on here that they will get sounds a great thing and should be given to everyone. Maybe the reason they aren't seeing each other so much just now is to try to remain celibate and not give in to temptation.
She sounds very sensible and bright and yes, she has a good job but becoming a housewife in a few years isn't a bad thing if that's what she wants, which it is.
Its her life and you have to let her live it. Iv definitely made choices my parents voiced their concerns and disappointment about. One, having my dd at 18. Two, getting back with my partner after we split up when I was pregnant.
However, we are now 30, married very, very happily with three children and he has a successful business whilst I this year gave up my job to stay at home. I can't imagine being any happier had I done what they wnated and I'd defo of had major regret if I'd had an abortion like my dad wanted and not had my daughter.
You can give her positive advice, but essentially she is an adult who is free to make her own choices and you just need to respect and support that. She most definitely isn't young to get married and I'm surprised you say it came out of the blue after them being together for three heard and being religious. Its the next step really for them.
I can understand your concern about them not having holidayed or lived together etc. But at the end of the day, even people in relationships who have done that, split up.