If i upgrade myself, can I pay for myself only and my friend remain on the board as a free user?

Hello, I currently use the free version of Trello and am considering upgrading to the business class. I have one 'team member' who is my Virtual Admin assistant. If i upgrade myself, can I pay for myself only and she remain on the board as a free user? Or, do i also have to pay an additional membership for her? I do not need her to have the upgrade version, but I do need her to remain on the board and to be able to continue to use the features she currently is using