My sister got a flat in one of the rear wheels of her car (FWD). Should both rear tires be changed out at the same time, or is it ok to just replace the one? Unfortunately, I'm not sure what to do when you have TWO rear tires on the vehicle. 

A speedy response will be much appreciated.

Replacing only the one tire will cause no problems.

Yeah, wherever you go is going to want to sell you two, but thats their job.  i used to work at a sears automotive center.  Just replacing one will be fine

Well, I'll say what (I suppose) every tire shop mechanic would say, i.e. change both tyres (left & right).

It doesn't matter IMHO whether the car is front- or rear-wheel-drive; it's always important to have tyres in the same treadwear situation as regards both sides of the vehicle.
Especially in FWD cars, the forward wheels undergo at the same time the "duty" to steer the vehicle and give it traction, but of course the rear ones are still very very important when it comes to the matter of roadholding in a curve/corner, and (IMHO again) the more the road is wet, dirty, etc. the more this is true.

Furthermore, I've heard some tyre sellers that recommend changing the rear tyres BEFORE the fwd ones, just because of this.

I know it's kind of a monetary PITA to get a single flat tyre and have to change two tyres,but anyway...safety first!!

(both on 2 and 4 wheels  )

I apologize for any language mistakes or similar (English's not my native language).

Lamps,
Simon.

What they taught us is that it's only absolutely necessary to change both if it's a flat on the drive end    We weren't actually allowed to change just one if it was a drive end flat ( insurance regulations )  But atleast down here in georgia we don't really have any tire stressing conditions  ( mountains and the like)

just wanted to add for precision's sake:

I mean, I'd suggest to change both tyres IF the flat one is so badly damaged it can't be repaired at all;
but if it's just a puncture or a small/minor damage that can be easily repaired...well, that's allright!

Spcterry:
Ok, I'm writing from Italy; here (almost) everyone would suggest to change both, regardless of the tyre being on the drivetrain or not; I didn't know about insurance regulations or similar in GA. Sorry

Lamps again,
Simon.

Yeah,  almost no shop in the US is going to suggest repairing the tire unless you ask about it   But definitely if the tire isn't badly damaged that would be the way I would go.   Those were just the regulations of the insurance company for the shop.  they didn't want to be liable for any issues arising from having unevenly worn tires on drive end.

Apparently the tire is damaged beyond repair, so that's out. Thanks for all the info. She's gonna just change the one tire. The car is basically only used for boring LA city driving, so it should be fine I think.

You should change both tires if: 
1.  They don't have the same size/make/model tire as the other one (something damn close will work too...)
2.  The other tire's tread is rather warn...  Warn tread can cause a slight tire-size difference, which could potentially lead to unwanted wear on both tires...  And on some cars/trucks could lead to unwanted wear on the suspention/ball-joint/etc/etc


But...  Once upon a time, I drove a car with 4 different brand tires (counting the spare) and 3 different sizes.     The tire guy asked me what size tire I wanted and I told him to pick what ever was on there already...  He looked at me funny and asked me which of the three different sizes I wanted to choose.   

If it has to be Changed only 1.Just me but if it's only a Nail or Screwdriver I would Plug it and Carry a Can of Fix a Flat.Or Fix a Flat it too and carry a Spare in the Trunk.
Right Now my Uncle has a Haybine that has a 15 Year Old Ford Escort Tire on it.It was ran until the Belts were Showing.It kept going Flat so I pumped it up and saw that it was full of pin holes where it had Ran over Rose Bushes when I dumped Soapy water on it.I went to the Autoparts and Got a Can of Fix a Flat and put in it to get us by until another Escort Tire showed up.To this day it hasn't leaked a bit of Air but he good one has to be Pumped up every week when we use it.

NO NO NO Fix-a-Flat !!!!!!!!!!
That crap can hurt someone when the tire is changed.
Just get one new tire and roll.

I'd say her driving habits should affect your decision. Does she drive like grandma? change one. Does she drive like she's rounding the last corner onto the checkered straight? change both.