You may not know what to say, but if you can show your natural concern in your own way and in your own words, the bereaved person will sense your sincerity.
Perhaps a card when the person arrives back at work with an expression of your sorrow. It is better to say something than nothing. You may feel awkward but if you avoid the bereaved person, you may add more pain to an already painful experience.
The individual may not be able to concentrate at work the way he/she used to and a temporary sharing of the workload will lessen the burden. If the individual works for you, give work that insures success and does not cause additional frustration. If the person is your employer be aware that he/she may be unusually short-tempered and that his/her work efficiency will change. Patience and understanding can assist your co-worker through this grieving period.