Remember to set up Activity Monitor properly before checking. You may know this but a lot of people don't, so forgive me if this is "preaching to the choir":
1) Don't check immediatlely after a restart. Some of the background culprits may not load unti after several programs have run. Doesn't hurt to check after the computer has been running several days.
2) By default, AM is set to show only "My Processes" and can miss some of the suspects if so used. At the top of the Activity Monitor window is a "Show" option:
Change it to "All Processes" as in the example, then highlight the "%CPU" column header to bubble all the heavy hitters to the top.
3) Once thus set. let AM run for a minute of two while you watch. Most processes cycle their usage and a screen shot or a two-second observation may not pick up a high-use process. That's why you may have to make a field trip.
This just occurred to me: How full is this person's hard drive? Printers generate temp files---some quite large--- during the print process. If the hard drive is short of space, it takes longer for these files to be written to the hard drive as the system looks for space.
Another thing: does your friend occasionally restart the computer? If not, lots of dreck can accumulate and eMacs can't hold much RAM. A restart clears an considerable amount of undergrowth--I try to restart our working Macs at least one a week.