My mother has pigeon-chest. What should I tell her?

Stay call. This a congenital deformity. Pt needs to keep active, avoid fumes & smoking (as everybody should), postural exercises. Be check by a pmr or orthopedic doctor to reviewed any other problem & if any severe problem & check a pulmonasry function to be on the safe side.
Not much. Besides appearing slightly different, it has little meaning in your life.