The tenant likely does have a defense against paying the higher rate of rent, and may be able to pay that rate for a third month before the 'corrected' rate of rent comes into play -- but the tenant needs to keep in mind that if this was a mistake, and it almost certainly was, the landlord is likely to at best have a tense relationship with the tenant going forward, and the landlord may choose to give notice ending the tenancy.
The tenant must thus weigh then benefit of getting a rent discount for two or three months against the possibility of having to move.
By the same token, the landlord should consider whether this is a good enough tenant to simply accept the consequences of his mistake and move forward.

