Check all the materials for out-gassing please. Highlight, click here.

This could be so many things and is something to take quite seriously. I presume some kind of building material is off gassing.

What climate does she live in and are the windows open frequently? If not, how does fresh air come in to the house? Can she intentionally leave windows open for as many hours per day as possible and see if this helps at all? Can set up a window fan to blow old air out and draw fresh air in?

Is it better or worse in some rooms more than others?

While she is figuring this out does she have any breathing relief from wearing an N95 mask indoors? I have environmental allergies often triggered by building materials. The silver lining of covid for me was discovering that I can wear an N95 indoors when certain materials are impacting my breathing and it makes a tremendous improvement.

Sorry no answers, just following cos I'm interested. There are "green" bldg materials & methods so ideally someone knowledgeable in this area would have designed the house and/or perhaps could be hired now as a consultant - they might know what the culprit is, or culprits plural - possibly its not one single thing. ETA: look for someone who is LEED certified (Leadership in Energy and Environmental Design)