Hi,
Is this user also user other Lync endpoint devices such as a mobile phone etc?
Please try to temporarily disable the user for Lync by using the Lync control panel actions menu on the users tab, confirm that the user no longer appears online (you might want to force replication of the address book if he is still appearing), re-enable
the account for Lync and see if the problem persists.
By temporarily disabling the account you will not impact the users contact list etc.
Completely removing this user from Lync would no doubt fix the issue, but would impact any database residing user data such as contacts and conferences etc.
Neither way would present you with the cause, but are probably the fastest resoutions.
Kind regards
Ben