You aren't going to like the answer, I'm afraid.
If she did an "over the air" (OTA) update, i.e. downloaded via the "Software Update" menu on the phone, then nobody can say why this happened (there are ways of updating using a computer, and some of those will reset the phone, but most updates are OTA). OTA updates are not supposed to erase the phone, but occasionally there may be a glitch or error. I can imagine different types of error that might result in this, but since all we know is that it happened it's impossible to know whether any of those was the actual cause. All I can say is that these things are rare, but you hear of them happening occasionally.
Now the bad news: if she didn't have any sort of backup (and it's worth making sure she didn't have a backup to Google or Samsung turned on without even realising it) there's very little chance of recovery. She could look for data recovery apps in the play store and see whether any of them can find anything, but I'm not terribly optimistic. A web search will find lots of PC utilities that claim to be able to recover data from a phone, but in all the years I've been here I don't remember anyone who wasn't obviously being paid to promote an app reporting success, so I'd not get too optimistic and at least look for a free trial before spending money (but if one does work do tell us - it would be useful to know!). Ordinary file recovery software you'd use to recover data on a PC will not work because the phone doesn't mount like a USB drive on a computer but connects using something called Media Transfer Protocol, and you can't run file recovery software over that (you could have if you were running Android 2.3, but that's 12 years ago). I expect the software that claims it can recover data from a phone will ask you to turn USB debugging on first).
A further complication is that the phone's storage is encrypted, which makes recovery more unlikely. This is where knowing exactly what went wrong would be useful to know, because if something triggered a factory reset there's very little chance: that will delete the encryption key, and at that point it's game over unless maybe if you are the NSA. Did the phone start normally but all her files were gone, or did it go through the initial setup process like it was a new phone? If the latter it might be that something triggered a reset, and if it did then it's probably hopeless.
You may find advice telling you that you can recover the data if you root the phone. In principle that does provide more options, but it's unlikely to help you unless you were rooted already. If you are in North America then most Samsung's there can't be rooted at all. And where a phone is rootable the process usually involves unlocking the bootloader, which triggers a factory reset, and that would be self-defeating.
I'm sure she's aware of this now, but if you have anything important on a phone it's vital to have a backup. I don't mean in case of a freak event like this, if you've no backup the data will be lost if the phone is lost, stolen, broken or suffers a serious hardware fault, and those things can happen at any time.
