Open case. Gently - gently - push finger against hub (not blades) of fan until it slows or stops entirely. Listen. Is the PC now quiet? It was that fan. Is it still loud? It was not that fan. Remove finger from hub and, if required, repeat process on other fans.
At idle it shouldn't be loud, no. Load, completely different story though. Yes, you could plonk in a cheap cooler like a Rajintek Themis or some such and it would be an improvement over the stock hsf... but is there a point? Try remounting the stock one with some fresh paste first and give the fan a clean. On another note, from other threads we know that you are looking to upgrade your own PC, so instead of buying anything for her PC, wait a bit and give her your old stuff once you upgrade?
A few months ago I had a similar issue with my wifes computer. Turns out the fans power cable had decided it was better off in the way of the fan blades than where it had been put.
Stock coolers from AM3+ chips are not really very good, and FX8350's can produce a chunk of heat. Is she using a passive GPU ? Also, if that's the only fan in there, it's likely to be moving warm air around the case and never really getting rid of it.
I use the fans provided with my fractal case, no complaints, but probably not the quietest out there - but quieter ones often seems to equate to more expensive with fans. If you download something like hwinfo64, then you can at least find out what CPU temp etc. is to determine if it's actually a problem.
