There are some good childrens books written at various levels (sorry don't know any in particular) that talk about different kinds of families, etc. that might suit the bill - for 5 year olds reading them a story that is fun is a good interactive thing plus it gives them a chance to ask questions or for you to add "like your friend "Sue" who has 2 mommies".

If your guess is correct she may want to know more about her why her friends family is different than the definition of "gay"