Again, having the receipt is the unusual part, especially after the vehicle has been sold. Yes, some people leave relevant receipts in the glove box, etc but most people don't. Then, how many new owners go through those receipts ? Even less. It was an uncommon situation. Some warranties do transfer between owners, some do not. Maybe Honda's doesn't ? End of story if that's the case.
All their 12V battery warranty states is the following:
“This warranty, unless changed by American Honda in a document provided to you prior to your purchase of a replacement
12-volt battery for your 2022 vehicle, applies only to replacement batteries (other than Hybrid Powertrain System batteries) purchased from an authorized Honda automobile dealer in the United States, Puerto Rico,
the U.S. Virgin Islands, Guam, and the Commonwealth of the Northern Mariana Islands.
Time Period
Replacement 12-volt batteries are covered for a 100-month (8 years and 4 months) limited warranty.
Warranty Coverage
During the first 36 months (3 years) of service, a defective replacement 12-volt battery will be replaced at no cost for the battery, labor, or installation.
For the remaining 64 months (5 years and 4 months), you will receive a credit toward the purchase of the 12-volt battery. This credit is based on the then-current retail price:
• Months37to45:60%
• Months46to55:50%
• Months56to65:40%
• Months66to75:30%
• Months76to85:20%
• Months86to95:10%
• Months96to100:5%
The 12-volt battery is warranted for the time remaining in the 100 months of the Replacement 12-volt Battery Limited Warranty. No cash reimbursement will be made. You are responsible for the labor or installation charges.”
So worst case they don’t get a “free” but rather prorated replacement, but I think the dealer is in the wrong here since there is absolutely zero mention of the warranty not being transferable and it clearly leaking acid all over the place.