How to let a client monitor the progress of a specific Kanban card?

It’s not possible to share just one Kanban card (or a select few) with someone, since Kanban Tool access is assigned at the board level.

Please invite the client to your account by sharing a board with them.
After they’ve joined, you will be able to email them a direct link to a specific card, by either assigning it to them, or adding a comment with a notification for them.
But do keep in mind that they will have access to the entire board where the task is located.

For more control, you can assign them read-only access, ensuring they can view progress without making any changes.