Well, as I said, I have always been very happy with Creative speakers. The model I have is no longer available, but something like
this looks quite cool, and, according to the write up, is supposed to sound good. Of course, I don't know what your friend calls 'mid priced', but you can get cheaper then these, and certainly more expensive! I think you can probably use her sound system speakers, by connecting the sound card to her hi-fi amp, but if the hi-fi isn't placed near the PC, this may not be practical, and if playing games, etc, you really need the speakers either side of the PC monitor.
As far as sound cards go, again there is a vast price range, from £5 to over £200.
There don't seem to be many about which don't offer 7.1 surround sound, but these should work just as well with only 2.1 speakers (the .1 is the sub woofer), and it does give the option of upgrading to a bigger speaker system later, if desired.
This one seems to be a fairly mid-priced example, again, you could go up or down in price.
I have just chosen what I might pick for myself, from one retailer. There are obviously other retailers to choose from, which might offer better choices or prices, but Komplett seem to be reputable and on the level. I have just bought a new digital camera from them, and the price was very good, without outrageous shipping costs.
I'm a bit out of touch with general PC specs of today, so hopefully Sandra, or someone who has recently built a PC, can offer you any further advice you need, but I hope this has helped a bit.