A "new" G4 is very old stock. If indeed it was factory sealed, then it had the problem from the factory--2-3 years ago.
It's not a matter of "they haven't solved it". They know what it is, and they are repairing phones out of warranty. But they can't repair a phone that they don't know about, and any G4 bought back in December was manufactured at least 18 months prior--and that was before they recognized the scope of the problem.
If he sends it in now, LG will repair it.
If he buys an LG G6, it won't have the bootloop problem. If he buys an LG V30 later this year, it won't have the bootloop problem.
LG has recognized the source of the problem and has fixed it, for current manufacturing and for repairs.
Yes, they've solved it.
I cannot find anywhere (include forums) or any source saying LG has really fixed it for any repaired G4 or V10. I just see massive amount of people report they send back G4, get a repaired one, then screw up again, then this forever loop goes on...
For V10, it could be different story but not sure yet, since most of the people like me just got repaired in last few months and even the problem may comes back again, it would be at least like 3, 4 months from now.