I do understand, in theory, why the OP thinks the GAC could be helpful. I am immunosuppressed, prone do severe dehydration (because of a different illness), and am in a
scooter or wheelchair (because of a 3rd illness). The only time I use the GAC is when I'm having hydration issues and need to stay out of the sun. I'm able to moderately control the risk to my immune system because I know my body and what it needs.
On the other hand, we should try to remember that every symptom has a range of severity. One person's immunosuppression can be much more severe than another's. If it helps for someone who truly needs it using that criteria, then have at it. The purpose of the GAC is to allow people to experience the park to the best of their ability.
Correct me if I'm wrong, but I think her issue could be that there is a difference in potential germ exchange when you're in lines vs navigating the park. When you're navigating the park, you are mostly in motion and are able to dodge people who are obviously sick. In line, you are kept in close quarters with the same group of people for quite a while. While in that constant close proximity, it may be easier to catch what they have. For the OP, this may be an issue. I don't know if a GAC can help with that situation, but it'd be worth asking.
I also respect the post above where someone said if everyone gets a GAC it becomes ineffective. I totally agree! It's as bad as all of those <ahem>bratty<ahem> teens passing the wheelchair around so they can get on rides faster. The system isn't perfect, but without the system, I wouldn't be able to ride many of the attractions at all. There will be misuse right along with those of us who truly need the system.