Transfer of property from the owner who is not available

One of my relatives has bought a shop from 'A'. This shop was jointly held by 'A' and 'B' with papers of the shop in the name of 'B'. But 'B' has in writing (a letter with signature but no stamp paper) sold his share of the shop to 'A' and withdrew his amount/claim. The case is 25 years old. My relative has been using this shop for last 20 years with no claims made by anyone so far. However, the only paper available from 'B' is that letter and he is unavailable at the address mentioned in the property documents and believed to be dead as his age on property papers is 55. 'A' is available and ready to stand my relatives claim. Now my question is how can my relative get this property legally transferred in his name without availability of 'B'.