2 life-long friends, Russian & Amer. corresponded. Does the Amer. party posses the to publish it?

1 - 2 life-long friends, prof. of Music corresponded with a written agreement to publish it eventually. 2 - The Russian friend passed away. 3- Copies of his letters became part of his estate. 4 - Does the America party has the right to publish this correspondence as is? 5 - Is this right exclusive or non-exclusive?