depending on where he lives, each state has their own laws, however, if it is written in the lease that the landlord can enter the premise without prior notice then theres nothing he can do. Most complex's will have it in the lease that they will notify 24 hours in advance, but thats more of a common curt thing than anything else. The landlords to have the excuse of "it was an emergency due to XYZ was broken and/or needed to be changed" so kinda screwed, but thats part of renting as opposed to buying your own house.