The death of a close friend is a major bereavement and can sometimes be unrecognised by others because it is not a formal relationship.
Usually you will know the identity of the deceased's closest friends and how they can be contacted. The address book of the person who died, perhaps the directory in their landline telephone or mobile are all helpful for this. The closest friends will usually be the gateway to other friends and acquaintances and may also be willing to help to tell others.