Replace a front passenger lower seat cushion or replace entire seat?

If the upholstery matches, change the whole seat with a used one. That's much easier than replacing just the bottom cushion.

Yeah, it's a pain replacing a cushion only, and expensive. I bought one from theseatshop.com, just the lower cushion and cover was over $200. You can buy an entire seat from a salvage yard for that much. Just start searching now for your replacement seat, because it will take a while for you to find the right seat in the right color. Be forewarned many salvage yards don't like breaking up interior sets, they'd rather sell both front seats rather than one at a time. Just be persistent. And if you absolutely can't find one in your color, they do make vinyl paints and leather dyes which you can find in any auto parts store.