So, apparently all I have to do is inform someone that he is dead. The problem is that the game won't show me where this person is to be found - the best I get is Riften. Can anyone tell me where I am to go and who I am to tell, I am trying to clear some of my back-logged quests.

There's a mill to the West of Riften. There are only two people who live there, Leifnarr's wife and son.

Heartwood Mill I believe. It's im the Rift somewhere, maybe someone else can describe the location better.

Was this a bidy found in a bandit cave east of Riften?

If so, there is a mill some way to the west of Riften, west of the large lake where the river bends. Go there and talk to the woman who thinks her husband has run off.

Thank you, thank you, and thank you.

Fair warning- that quest sometimes gets bugged if you find his body before getting the quest to find him. The dialog option for telling the wife never comes up.