Do they have a sump pump? If they do, they should check this out. HEPA filters can be purchased at Home Depot, Lowe's etc. they are free standing filters that remove 99% of all allergens and contaminants. This will remove the musty smell. How old is the house? Do they have a basement? If so, there may be leaking water down the walls. Have they checked the outside lines? We had a broken pipe for a year before we realized it.