In this situation, the "win-win" solution is to present the options to the "customer" and let them make the decision. I do this all the time with my adult kids. Depending on the circumstances, sometimes I push the better parts (difficult, time consuming labor/critical part). Other times, they choose the "less better" aftermarket parts, again, based on the specific circumstances.
In your case, if the lifetime NAPA part goes bad in 2 years, the customer still feels good that they only have to cover labor, IF it does go bad.
In any case, make the customer choose so you are not blamed (no good deed goes unpunished).