Well plugs don't get changed for 100,000 miles. But more importantly the work should be guaranteed for 30 days, so have her go back in and get it fixed.
BTW, why do you have the error codes? Did you pull them or did the shop tell you? And if the shop told you, why didn't they fix it, or are they just as confused?
There are some pretty common scenarios where Xterras have had knock sensor errors. Do a search and you should find some info.