DO be available... to listen or to help with whatever else seems needed at the time.
DO say you are sorry about what happened and about their pain.
DO allow them to express as much unhappiness as they are feeling at the moment and are willing to share.
DO encourage them to be patient with themselves, not to expect too much of themselves and not to impose any “shoulds” on themselves.
DO allow them to talk about their loss as much and as often as they want to.
DO talk about the special, endearing qualities of the person they've lost.
DON'T let your own sense of helplessness keep you from reaching out.
DON'T avoid them because you are uncomfortable (being avoided by friends adds pain to an already painful experience).
DON'T say that you "know how they feel". (Unless you've experienced their loss yourself you probably don't know how they feel.)
DON'T say "you ought to be feeling better by now" or anything else that implies a judgment about their feelings.
DON'T tell them what they should feel or do. DON'T change the subject when they mention their loss or their loved one.
DON'T avoid mentioning their loss out of fear of reminding them of their pain (You can be sure they haven't forgotten it.
DON'T try to find something positive (e.g. a moral lesson, closer family ties, etc.) about the loss.
DON'T point out “at least they have their other ...”
DON'T say they “can always have another ...”
DON'T suggest that they “should be grateful for their so-and-so...”
DON'T make any comments which in any way suggest that their loss was their fault (there will be enough feelings of doubt and guilt without any help from their friends).