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The who's online block is updated using a garbage collection routine, meaning that once in every 10 vitits, the system checks which sessions are live, and which aren't, and updates the block accordingly. That's why your user stays 'online' for so long.
At the moment there is nothing you can do about that short of hacking the garbagecollector routine (and I don't know where to point you for that), but I'll ask the devs to look into this.
Herko