Master and Slave settings are necessary on an IDE (aka PATA) port / cable simply because they are shared between TWO devices. Some people are confused and think the Master has to be set up for the machine as a whole because that is where it boots from. Not true, although almost always the boot drive is set up as the Primary IDE port's Master device.
Whether you have one IDE port or two on your mobo, EACH can support two devices. On each port, IF it is used at all, there MUST be a Master. If there's a second device on that same port, it MUST be the slave. It also is best practice to arrange that the Master device is plugged into the connector at the far end of the cable, and the Slave in the middle.
On the ribbon cable (for data) note that one edge will have a red stripe along it. This edge must go to the #1 pin on the mobo connector, and same at the drive end. Both mobo and drive should have labels on them indicating which end of their connectors has Pin #1. In addition to that, many connectors have either extra bulges (and matching cutouts on the socket), or a blanked-off pin hole, so that you can only insert it one way.
Ribbon cables can be had with only two connectors - one at each end - to support only one device, but the regular 3-connector ones are to be preferred. Even if you only have one IDE device, it preserves the option for future changes. The connectors on each end will have 40 pin holes - in a 2 x 20 rectangular array - or maybe have one of these blanked off. The ribbon itself, however, should have 80 wires in it to work at the faster speeds (the older ones had 40). To check, start counting across. If you get to 20 and you're only one-quarter across, you gave an 80-conductor ribbon. Connectors do not have to be used, so a three-connector cable with only one IDE device and an empty middle connector is just fine.
Look on the device - in your case, your wife's hard drive - for markings next to the jumper set. These tell you how to set the jumper. Normally you have choices of: Master, Master with Slave Present, Slave, and Cable Select (CS), but you may not have all of these. Set one device to Master (or Master with Slave if appropriate) and plug into it the end connector of the IDE data cable. Set the second device for this channel (if you have one) to Slave. The alternative, if BOTH devices support it, is to set BOTH to CS and let the machine sort it out. In this option the device on the END of the cable WILL be the Master - that's set by the cable.
If you have both a hard drive and an optical drive on one IDE channel, it would be better to set the hard drive as Master. Some optical drives are OK as Master with a hard drive as Slave, but some are not "smart" enough to do that.
Harvey is right, you can do all of this as a temporary setup if all you want to do is copy files from your wife's drive to yours. Not even mounting it inside your case is OK. I've even put the temporary drive on top of the open chassis, making sure there was a sheet of paper or something under it to prevent shorting out the drive's PC boards on the metal case. Like he says, don't drop anything!
