The disciple is only bound to serve the guru as long as the guru is addicted to Krsna. Even if the guru is tainted by some conditioning, if Krsna is visible through them, then they should be served sincerely.
The disciple should only be concerned with Krsna - not sentimentally attached to the 'celebrity' of the guru. Therefore, even if the guru falls, then the disciple should simply be concerned with Krsna and never lose sight of this. In such a state one can again take shelter of a guru through whom Krsna is visible again.
The honus of understanding the symptoms of what it means 'to be addicted to Krsna' is on the disciple. These symptoms are defined and should always be seen in the guru. In this way, the guru will take the disciple Back Home, Back to Godhead. The disciple also has to be suitably qualified to be able to recognise these symptoms of purity in the guru - it takes one to know one.
