One would want to review the letter and to know what potential causes of action the corporation is worried about. In it's current form the question is just too general. Most causes of action belong to the estate of the individual who is deceased, subject to applicable statutes of limitations and other factors.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
I'll jump in here and add a penny, after reading the comment to Mr. Doland’s answer. The heirs (administrator of the decedent's estate) can have no greater rights than the decedent. I am not certain, however, that the letter you reference limits the decedent's rights as you suggest. You should consult counsel who can review the facts and the purported shareholder agreement or estoppel certificate to determine its value and applicability.
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Attorneys Daymude and Doland have provided you with some very good advice.
Mr. Crosner is licensed to practice law in California and has been practicing law in California since 1978. The response herein is general legal and business analysis.. It is not intended nor construed to be "legal advice" but rather it is analysis, and different lawyers may analyze this matter differently, especially if there are additional facts not reflected in the question. Oftentimes the question does not include significant and important facts and timelines that, if known, could significantly change the reply and make it unsuitable. I am not your attorney until retained by a written retainer agreement signed by both of us. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.