Condolence cards should be sent whenever a person close to you experiences a loss of a loved one. There are a number of things to consider when following proper condolence card etiquette, the most important of which being timeliness.
Sending a card of condolence can be tricky as the recipient is likely to be in a sensitive and vulnerable state of mind. While timeliness is the most important consideration, there are other things to think about that are almost as important.
The first of these is at whom the condolences should be targeted. If the sender knew the deceased but not the family, common practice is to send the card to the closest family member. If the sender did not know the deceased but knows the family or individual, condolences should be sent to the surviving individual.
Another consideration is what to say in the message itself. Unless the sender is exceptionally close to the deceased or the recipient, the message should stay short and simple. The recipient is likely to be inundated with sympathies and a long letter from everyone could overwhelm. Good examples of short and simple condolences are "I am sorry for your loss" or "Deepest sympathies to you and your family."