My youngest daughter's best friend has an allergy to cats. We do what everyone else has already suggested and we vacuum, dust and we wash the slip covers on our furniture, change all the sheets and blankets on the bed where she will be and we also keep the cats out of that room for a few days to a week before she comes over. When she is here, the cats remain downstairs,which is a big area and is comfortable for them. They don't seem to mind. She did get a little bit of a reaction when she slept too close to the window. The cellular blinds probably had a bit of cat dander on them, but other than that she does well. Her mom is surprised how well she does actually because she doesn't do as well at her other friend's houses. Poor girl has only cat lovers for friends!
I don't know why she does better at our house. It could be that we had a lot of uncarpeted areas compared to the others or that we use our robot vac every day. Or it could be the diet of our cats on an all grain free canned/raw diet. Her other friends cats eat low quality dry food. I read a study about that once so I am curious whether that is part of it or not. I also read that unneutered male cats can be a big problem to allergy sufferers but again, I don't think that is proven either.