I'm working on the idea that what your friend wants is to dampen as much sound as possible, without a huge monetary layout. I've had great luck dampening walls by just installing either fiberglass or cellulose insulation in the wall cavities, then installing a layer of drywall, followed by a second layer of drywall with 1/2" furring strips between the two. You can simply use Locktite power grab to glue the 1" x 1/2" wooden furring strips every 16" to the first layer of drywall, the screw the second layer onto those furring strips. Same technique for ceilings. Be sure to tape and float both layers of drywall. This will dramatically reduce sound transfer at a very reasonable cost. A solid core exterior door, or better yet, a sound deadening door should give great results. Two sound dampening doors, one at the top of the stairs and one at the bottom is worth the extra cost IMO.