I think you need to sit her down and explain the difference between her home and yours, and why some people would be repulsed by her home, scared for their children's safety due to germs.
I have always been quite repulsed by the same sort of thing you are at your friends, I wouldn't drink a tea at a certain friends house due to a similar problem, though not as bad, more kids hand print marks on glass that had never been cleaned, and dirty surfaces due to kids, that had not been cleaned for a long time, that kind of thing.
I noticed with my children there is a difference in them also, the youngest always had been clean and "got" germs from a young age, the oldest oh dear, well a lot of educating is still going on there, and I think that there are just some people who don't have that natural repulsion of germs that most people have. You know the not wanting to shake hands with someone sneezing that type of thing, it simply would not occur to my oldest the link between the germs and spreading and she is very bright, it is like there are links in the brain that are not working, she has it with some other things, like she could not tell the time on a clock that was not digital, then she learned to tell the time in Italian, and it finally clicked, and she can tell the time now she just has to translate it to English so takes her extra time, she is 13, and nothing worked before regarding telling the time, it just didn't compute in her brain properly before!
Enough waffling!
I really think you need to go over the basics of germs, go over, how often people in general clean, how they clean and why they clean, a few video's of how clean is your house as someone mentioned earlier may help her.