I would check the task manager first and see how the memory consumption looks. If that checks out I would try the following. This is assuming that the users mail file is able to be opened from other machines/clients.
– Exit notes and delete cache.ndk, relaunch and see what happens. If that fails exit notes again and rename bookmarks.nsf to bookmark.old (or whatever you prefer). Relaunch…..
Those are just a couple of easy things to try, maybe one of those will get you by….