Please don't follow the advice of KarlG on this one. Do NOT delete the user's bookmark.nsf file. This should only ever be done as a last resort, and when the issue has to do with a link on a workspace or in the bookmark.nsf database.
This is a link received in an email and has NOTHING to do with a bookmark issue. It is likely and issue with access rights. The receiving person does not have rights to the sending person's email/calendar. To give the receiving person rights (if this is actually necessary) have the sending user open Tools-->Preferences from their inbox. Go to the Calendar & To-Do tab, then select the Access To Your Mail & Calendar tab and add the receiving user to their list with read only access - that should solve this issue.
You should have the user recreate the bookmarks. The old db is corrupt, so its items are of no value, although making a copy is probably a good idea in case this turns out not to solve the problem.
You’ll have to close down the user’s Notes client completely, then delete the db from the “Data” directory via Windows Explorer. When the user signs back on, Notes will create a new bookmarks db, and the user can open each database and create a bookmark for it. It’s a major inconvenience. About all you can do to simplify it is take a screen shot of the bookmarks list before deletion so you know what to replace.