My husband joined the Navy right out of high school, we were cute 18 year old babies, and his mom was devastated that we'd be breaking up (I was his first real girlfriend), and we had no idea what she was talking about because we had no intention of breaking up! And now we're married, so it obviously worked out
So bootcamp was pretty awful to go through, we wrote each other every day--they're not given much time to write but mine found a way to write me every day (I think he had to hide a stash of letters in his pillowcase or something, lol). I don't know if it's different now, but in 2008 they could only mail letters like once a week, and sometimes they'd miss it for one reason or another. He always apologized in his letters about the delay I'd get in receiving mine, but begged me to send one EVERY SINGLE DAY, and to please stuff it with pictures of memes ahahaha. I'd check my dorm mailbox every day, and there would be nothing until one day there'd be like 10 envelopes crammed in there and I would read them all in like 5 minutes and cry bwahahaha. The phone calls are VERY limited and sometimes he called his mom instead of me and I'd be mad about it, but he missed his mom, too!
After bootcamp is over, they can video chat every day and it's fine. It's FINEEE. Long distance relationships are easier than ever now! We had to use dinky ol' webcams back in 2008
I still roll my eyes when my friends complain about a spouse being "gone all the time" and I'm like HELLO WE SPENT AN ENTIRE YEAR APART ONCE and then they roll their eyes back at me, lol. Deployments are worse than bootcamp, though. That's a whole 'nother book.
ANYWAY, TLDR; if they both want it enough, it'll work. If not, it won't.
Just be there for her and try not to worry about it and let her vent to you! My mom was not great when I'd want to talk about my feelings, so I had to find support groups on facebook to talk to other girls in the same situation. Nobody else gets it. Finding other girls my age in the same situation helped tremendously!