Deceased beneficiarie's husband can't be reached to sign agreement for Trust settlement

All the beneficiaries have signed a Trust settlement agreement except one. The person left to sign the Trust Settlement Agreement is the husband of a deceased beneficiary. According to the Trust attorney they have tried to reach him several times and he has been completely non-responsive. Because prior to the Settlement Agreement being circulated to the beneficiaries the husband authorized the Trust attorney to make the settlement offer (and the Trust attorney has his signed authorization ) The Trust attorney suggests that the other beneficiaries simply agree to attach the husband's authorization to the settlement agreement and proceed as planned... My concern is that the husband could pop up sometime in the future, after the Trust is settled (and the Trust attorney and Trustee are long gone) and renege on the Authorization he signed and come after the other beneficiaries...