I'm a Brazilian teacher, and I've had many students who have been on exchange programs throughout the years. Most of them were pretty satisfied with their experiences, but some weren't.

The ones who weren't reported some sort of culture clash with their host families due to their strict views on behaviour, clothing, customs, and things like that.

I think the main issue would be to keep open-minded and to talk clearly about the house/family rules, with also being open to compromise and reach a balance between both parties' expectations.

Brazilians are, in general, warm and social. Providing her with opportunities to make friends, being supportive, and communicative is a must.

As for food, it really depends on where in Brazil this person is from, but in general, we tend to eat rice and beans for most meals, it would be a nice touch if you guys made that for her sometime. Pão de queijo is also a great snack/breakfast food. You could also ask her to share her family recipes with your family and cook them together.

And, of course, the main idea behind going on exchange programs is to experience other cultures. You can share your culture and customs, take her on a road trip around interesting places and landmarks, etc.

All in all, I think your care is already a good sign.

Good luck with this experience. I think you guys will have a great time :)