My Mother has bad breeating problems and a friend of mine suggested I have a uv light with peroxide for the ac unit installed to help her. Does anyone know about such a unit and do they work? What should this cost?

Not real sure what they are meaning about with peroxide, but a UV light would be an excellent idea. The UV light will kill molds, allergens, and even some airborne viruses. A yearly cleaning of the evaporator coil will also help with this. Clean the condenser yearly to help out with efficiency and extend the life of your A/C too. You might also have the ducts inspected to see if they are dirty or leaking. Install a good MERV 11 air filter on the system also. Check out the Ultravation website for more info. If the home is one of these newer super insulated homes you will also want to look into getting an Air to Air Exchanger

Maybe they were thinking Ozone O3 not peroxide O2. UVarcs produce Ozone but it is not usually found in UV lights. O3 is not something you want in a breathing environment.