My youngest sister suffers from both of these “illnesses”. They can be scary sometimes. Asthma for obvious reasons, not being able to be breathe and all. Allergies can also be dangerous if not taken care of.
Grandma G. was concerned when they moved into their new house because of the possible dampness in the basement. Dampness encourages the growth of mold and mildew, which she is allergic to. Severely allergic…
While they have a standard “space” dehumidifier, they could also look into a whole house dehumidifier. Santa Fe dehumidifier makes larger models that will keep whole houses or buildings free from moisture. I don’t know that Grandma G. would need something like this, but t’s good to know all your options!! 
