Thanks for the update, this sounds much more plausible then the resetting of the user mapping on the cluster.  It sounds like the client is connecting to their AD credentials properly on one network, then authenticating as a guest or alternative account on the other method.  Hopefully this gets worked out quickly!
I was asked to reset a users token on the Isilon because they was unable to access a share they once had access to.  I was trying to find the current syntax to perform such a task?    isi auth mapping token???
How to reset users token on Isilon?