Going to talk 3rd person to avoid potential issues, so just follow along.
She purposely bought a bootlooping LG G4 on eBay to see if LG will fix it. She called LG at 800-243-0000, got through the phone menu system, and talked to a real person. LG explained that without a "proof of purchase" (receipt/invoice), they initially had the warranty at 15 months from manufacturing date, but they extended it to 18 months. With a proof-of-purchase, the warranty is 12 months from purchase and does not change. She told them her IMEI to check and it was 1 week past the 18-month mark, so they said she can send it in but they'll most likely tell her how much it costs to repair or they'll ship it back (free FedEx both ways). They refused to comment on the cost of repair. Compared with the data retrieved
here, LG's manufacturing date was a 1 day earlier than the 'ESN Date(Prod. Date)'. LG explained that the device must not have physical damage, water damage, cracked screen. She
made a RMA with an invoice from another eBay G4 purchase from under a year ago and sent it in. The invoice doesn't indicate new/used. She sent it to LG and LG is repaired it as though it was under warranty. She got the phone back a few days later and it is fixed. The phone was sold as SIM unlocked, but now, it is SIM locked.
TL;DR: If you bought the phone more than 12 months ago, call LG, tell them you can't find your proof of purchase, and maybe the manufacturing date is within 18 months. If you like to "work the system", just don't screw it up for everybody else.