He also said
'm excited about the new battery but Nissan says it will only have a 12 month /12,000 mile warranty on the replacement battery, not the continuation of the original warranty
Which makes no sense. This contradicts common sense and what's in the warranty booklet. Replacing it shouldn't shorten the warranty but won't start the 8 year/100K clock over either. Worst case is 8 years/100K, whenever it originally ends... or if it were on the last day/mile of that warranty, he gets 12 months/12K miles additional.
From page 26 of the '16 Leaf warranty booklet I downloaded:
WHAT IS COVERED
Nissan2 warrants to correct defects in materials or
workmanship in all genuine Nissan replacement parts
and Genuine Nissan accessories, distributed by Nissan
North America in the United States, installed and used
on Nissan (if an appropriate use and application of the
part or accessory) vehicles only, except as described
under the caption below, “WHAT IS NOT COVERED.”
This warranty covers any repairs needed to correct
defects in materials or workmanship.
HOW LONG IS THE WARRANTY
Except for replacement audio components, this warranty
is for 12 months or 12,000 miles from the date of
installation or purchase, whichever is earlier. However,
the warranty on Genuine Nissan replacement parts and
Genuine Nissan accessories installed during the Nissan
New Vehicle Limited Warranty will extend through, and
will not end before, the end of that original warranty.
REPLACEMENT AUDIO COMPONENTS
A replacement Radio, Amplifier, Navigation Unit,
Bluetooth, Control Unit or Compact Disc Player/Auto
Changer supplied by Nissan is covered for 12 months
from the time of installation or the balance of the Basic
Vehicle Warranty as it applies to audio components,
whichever is greater.