Tires: one tire scrapped, what to do?, tread depth, scrapes

Question
i noticed recently that my wife has a quarter sized fillet that was peeled from her sidewall. judging by the scrapes in her wheel, i would guess that she curbed the tire, or something similar to that. she is losing roughly half of her air pressure per 24 hours. these tires have an average of 7/32nds of tread left on all 4 tires. at this point i know that tire is junk, but what about the others? what to do? what would you do in this situation? i have pretty much come to grips with the fact that i will have to purchase a new set, but i am just not sure if that should be the way to go. obviously, i would rather find a more cost friendly route, but safety is definitely my #1 concern. also, we live just south of cleveland, and winter is approaching(cleveland winters can be nasty). actually, after writing this out, i have pretty much convinced myself that i should just go ahead and order a new set. i suppose at this point i am just wanting to hear an expert tell me to do so, so i know my money being spent is justified. thank you for your time. take care.

-mark

Answer
Mark,

In this case, I think it would be OK to replace a single tire with the same identical tire.  The difference in tread depth isn't enough to worry about.

I would be better to replace a pair and put the new ones on the rear.

And best of all would be to replace all 4.