If they invest via joint ownership, their joint owner will succeed to their ownership in accordance with the terms of the joint ownership.
The C-shares are not transferrable whatsoever under the terms of your Bylaws, which the disclosure statement states. Upon a member’s death, the Co-op can elect to repurchase them and the proceeds would go to their heirs in accordance with their will.
Under the terms set forth in the disclosure statement, the Notes may not be transferred without the Board’s approval. Transfers pursuant to the deceased member’s will would be subject to the Board’s approval. If the Board intends to approve transfers pursuant to inheritance, you can inform the members of that.
If there are questions beyond that, you should let them know that they should consult with their personal financial / estate advisors.