There may be an easier way to do this...but this is how I'd do it.
Use last to find the host the user in question is logged in from. Then go to that box and use 'ps' to determine who is telnetting to the box in question. If there is more than one person, The BOFH way would be to kill each process until the user in question mysteriously disappears from the target machine, but you probably dont want to do that. You can probably use a bit of common sense and login times etc to guess. Repeat this process until you reach machine A and the real username.

If the user has logged out it makes it more difficult.