Actually, it's more like 1-10%. Depending on the OS, and how much virtual memory you need, you should keep somewhere between 1-10% of your drive free for virtual memory and temp file use.
Another way to look at it, is to keep 1-2GB of space free at all times. Regardless of drive size, since the percentage thing is back from the days when drives were only a few hundred MB. However, 1-2GB of free space should be plenty for swap and temp file purposes for probably 99% of desktop users.
I'd say the person should focus more on making sure they have no viruses or other types of malware, and possibly consider an extra 512MB of RAM. Some store, online or local, is bound to use the upcoming 4th of July holiday as an excuse for a sale to try and get people into the store. If our friend is from Canada or the UK, then they may have to wait for some national holiday in their respective country.