NEC 2008 400.22(E & F) For lamp cord, the following conventions are followed:

Polarized solid black, white or brown insulation
Ribbed, grooved or striped side is grounded (neutral), smooth side is ungrounded (hot).

Clear insulation lamp cord
Silver conductor is grounded (neutral), copper conductor is ungrounded (hot).

Grounded (neutral) goes to the threaded shell of the lamp and is wired direct without break to the wide blade of a polarized 2 prong plug or in the case of a suspended lamp, to the grounded (neutral) white wire. Switching goes in the ungrounded (hot) side. You never want the thread shell to be hot (switch in off position in the neutral side with intact filament).

If the conductors are both the same color, the insulation is a single color and the printing is on one conductor's insulation only without rib (feel it with your fingers) or stripe, then that will be your continuous marker and grounded (neutral) indicator. The whole idea on zipcord is that you have some definite identifier as to what is grounded (neutral) to keep a continuous connection and wire properly to the lamp socket and wall plug.

National Electrical Code

ARTICLE 400 Flexible Cords and Cables

II. Construction Specifications

400.22 Grounded-Conductor Identification. One conductor of flexible cords that is intended to be used as a grounded circuit conductor shall have a continuous marker that readily distinguishes it from the other conductor or conductors. The identification shall consist of one of the methods indicated in 400.22(A) through (F).

(E) Tinned Conductors. One conductor having the individual strands tinned and the other conductor or conductors having the individual strands untinned for cords having insulation on the individual conductors integral with the jacket.

(F) Surface Marking. One or more ridges, grooves, or white stripes located on the exterior of the cord so as to identify one conductor for cords having insulation on the individual conductors integral with the jacket.

Note: National Electrical Code (NEC) is the US standard and quoting it defines this answer as being for US electrical power. If your country differs, feel free to post your own answer with your country or jurisdiction identified.

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.