How would you counter someone who's written a bad story that's full of poor spelling and grammar, telling instead of showing and illogical plotlines and who thinks that what makes a tale good or bad is only based on opinion, not facts?

What makes a good story IS based on opinions not facts unless you are looking into a non fiction/educational book.

Unless they are asking me to proofread or for my specific opinion, I'd say nothing at all. Life is too short to police others literary contributions.

Why counter them?  They arent going to sell much if its bad and that will either prove your point or it wont.  If the person is so self-opinionated that readers reactions dont change anything then you arent going to change them.  And there are several things that separate good writing from bad.  Many of which you mentioned.  Any author who thinks they dont need good grammer and spelling, strong characters and a cohesive plot, among other things, is a fool who doesnt really want to make sales.    Theyre usually the sort to inflate their books with fake reviews and go trash other authors who sell better than them