Why does this switch throw the breaker when turned on?
If your hypothesis is correct, which it most likely is, that the switch controls one of the outlets, then the extra pair of wires in that outlet have to do with the switch. Either they provide a separate loop to control just one half of the outlet (some switched outlets are like that: a switch just controls one of the receptacles out of the pair). Or else the extra wires are meant to be in series with the outlet, going out to the switch.

It sounds as if you miswired it so that the switch hookup is in parallel with a receptacle's hot and neutral (thinking that it's a daisy chain connection to the next outlet).

When you do that, guess what happens. Everything works when the switch is open, and when you close it, it short circuits.

A half-switched outlet, if that's what your friend has, normally requires three connections (not including ground). Two wires will come from the breaker, and two from the switch giving you four. This means two of them get connected together. How this happens is that the switch receives hot, and so it screwed to the same terminal as hot, or otherwise spliced to hot. The return wire from the switch feeds the secondary hot that controls the switched part of the receptacle.

The best way to proceed here is to turn the power off at the breaker and verify continuities: in particular, which pair of wires corresponds to the switch. With the power on, you should also check that you haven't wired any outlet with the opposite polarity: neutral being hot and vice versa.