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I am sorry to learn of what appears to be a rather unique issue that you are dealing with.
In order to dictate a decorum or dress code of sorts (even if it is simply to "dress appropriately") your organization would either need to amend its governing documents, or pass a board "rule" using its rule making authority to create such a rule (this would fall withing the board's rule making authority to govern the conduct of meetings - similar to allowing organization members limited time to comment, etc.).
But you would need to ensure that it is a rule that applies to all members, and is not a rule or mandate directed only at one person (even if you are forced to pass such a rule due to that one individual).
As with all other rules or amendments, check your organizations by laws or other governing documents for procedure. Each organization has its own procedure, and you must follow yours.