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Try: menu, settings, applications, then scroll down till you find the market and select it. Then choose clear data and reboot. See if that works. If not, try deleting your Google account, restart your tablet and enter the account info again.
He might also need to go under the security settings and check "Unknown Sources" to allow the installation of non-Market apps. If this is not already selected. Not sure if this will help as I am not aware of exactly what that error message entails.
Todd