Thanks everyone.

Yes, we do have a tenancy agreement.

Okthen,

If your friend has reached the end of existing agreement, then the landlord has the right to change the rent in anyway s/he see fits. Your friend could always decide to move out if it's not acceptable.

Being both a landlord and a tenent, I've come to the conclusion that everything I do as a landlord is always reasonable, but anything that my landlord does is inevitably unreasonable...

Ed