Problem connecting to Wi-Fi

Try resetting network settings and rebooting. That will mean she will have to reenter network passwords for all her saved networks. But then, that's what you're trying to get anyway.

 

Restart the router and check again.
Also see if the iphone connects to other wifis or has problems connecting just to that one network.

 

When something worked fine for ages and then suddenly goes bonk it is often software.

Do as previously suggested and try other wifi, passworded and non and other devices on hers. If that all works out then software is most likely the culprit.

Make sure she has her stuff backed up both in a backup and photos etc separate in case the issue is one that carries into the backup. And have a computer handy. In fact, even if she's doing iCloud backups do one to said computer as well.

Start with a reset of the network settings. If that doesn't work, restore the iPhone and see what happens. In fact, do it as a DFU since it's a core function issue. Might be overkill but it's only perhaps five minutes more work so why not. If it is a dodgy wifi chip/antennae sometimes they will finally burn out right as you do the restore and no wifi will come up for activating the phone. Thus the reasons for the computer. You can activate and restore using it. And you will typically see a greyed out wifi switch if it is burned out. If you are still under warranty for sure hit an apple store. If not you might still to see about an out of warranty replacement or, depending on the age of the phone, perhaps it's time to upgrade.