Rent hike at apartment complex?

i have a friend who lives in a extended stay hotel. its really an apartment complex tho because he pays monthly. the office wants to raise his rent even though he has been paying 600 bux every month for the past 2 years. im not sure if he has a written or oral agreement but i need to know what he can do to block this hike and if he cant what procedures need to be followed to properly inform him that his rent is going up? i.e. is a certified letter needed?