My job is cars. Our rule of thumb is that the first step is always to catch up on maintenance before doing any other diagnostic tests. On your car, the spark plugs were due to be replaced at 100,000 miles. So if you have not done that, then just bring it in to have them replace those. Unfortunately, because you have a 6 cylinder car the labor could be as high as 1.8 hours according to my labor guide. In terms of other diagnostics, the only thing worth having them do would be to ask them to do a quick "wet test" of the spark plug wires and the ignition coil pack. If they get any arcing, then those component(s) could be an issue.
If you have more questions, feel free to PM me. You can send me the engine codes from their diagnostic if you want.
I also need to correct some misinformation. If the only thing the repair shop has done for her was to read the codes stored in the computer, then they can NOT tell whether it is the spark plugs, ignition coil, ignition wires, or other component. All it will tell them is a code number or numbers. Misfire codes could be any one or more of codes P0300, P0301, P0302, P0303, P0304, P0305, P0306. Also, if you decide to replace your own spark plugs make sure you research the exact recommended spark plug. Some cars require a fancier plug than a $3 Auto Zone plug or you can cause bigger more expensive problems by using the wrong brand or type of plug...seen it many times.