Hi Leonardo, great question! This link sums it up well: https://help.trello.com/article/824-sharing-links-to-cards-and-boards
You can share a link to a board, card, comment or action in Trello. If you are linking to a private item then the person you are sharing the link with must be a member of that board in order to open the link. Links to team visible items can be opened by members of your team but one will still have to be a member of the board to collaborate on the linked item. A link to a public item on a public board can be opened by anyone that has been given the link.
You could possibly copy your card to a board in which the individual is a member, or to a public board so that they can see the contents.
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