Yes, that is an excellent point that I forgot to mention earlier.
Manufacturers rarely extend warranty coverage, and the idea of them extending it for an extra 6 years is just not possible.
Even if this repair was needed during the period of the Powertrain Warranty, it wouldn’t have been covered if the OP’s sister failed to change the oil at least every 6 months. Can the OP verify how often–in terms of elapsed time–her sister had this engine’s oil changed?