In most apartment complexes you must designate certain apartments hadicapped accessible. These apartments have wider doorways and halls to allow for wheelchair access, lower cabinetry, handicapped fixtures in the bathrooms, lower light switches and no stairs, among other things. After this, if a handicapped person chooses to rent a non handicapped designated apartment then he or she can. The owner can not refuse reasonable construction changes to the apartment to make it handicapped accessible; however, they can charge the person living in the apartment in order to make them. As for the stairs, you may have an argument with that. If they are saying that the building met ADA standards then it needs to have a ramp or a lift to allow access to handicapped persons. Most newer buildings/ remodels MUST be up to ADA standards. If there is no ramp, cut out in the curb, etc. to allow him wheelchair access, then that is likely on them cost wise to fix.
You say in your first sentence that the apartment is handicapped accessible- is this something the Landlord claimed? If this is the case and your friend can prove that it is, then the owner is responsible for the costs of all changes. The person responding below is correct about that. Otherwise, I stand by what I said above.