1) Ask your grandson what he did to the phone. If he flashed a different ROM, have him flash the stock ROM back. If not, ask your local AT&T store where the Device Support Center is. (It might even be in the rear of that store.) Ask at the DSC if they can get you going again. At best they'll reflash the ROM. At worst they'll have to replace the phone.
2) It's possible that the phone got an over-the-air update while he had it, and the update went bad. If it's a bad update, it's AT&T's responsibility (the law holds that whoever broke it bears the liability, and they sent the update, so they broke it) and I've had them replace a few phones for that if they couldn't redo the update - they know how it works, and they don't argue about it. (The last time, they apologized for having to replace my phone. If my data wasn't all backed up I would have lost it. But you should always have any important files backed up anyway. There's no telling when the phone gets dropped, driven over, lost, stolen, etc.) If the update could be redone, I've done it myself, having been in the business too long to waste the gas and time to bring it to them to do what I can do in 5 minutes, but sometimes the update goes so bad that it causes hardware problems (like the battery dieing in the middle of an update). I don't know the current LG line, but 10f sounds like an update from 10d. (Which means it could have been over the air or someone could have tried doing it manually and messed up by updating with a Korean update file.)
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