There are no limitations on sharing boards between people on a free plan and a professional plan. Free users can view any content created by a professional user, and content shared with a free user doesn't count towards their card limit.
The only restriction a free user has is the number of cards they can add based on the free plan limit.
Example: a designer at a creative agency might share a board with their client to ask for feedback. The client can view any content on the board, and add up to 100 cards (generally more than enough for this type of collaboration).