So that quest, Blood on the Ice. On my secondary character it wasn't bugged until patrolling the streets now on my main. I can't buy the house or start the quest because it's such a bugged quest. So, I guess I also can't do it by trying to find an alternative route into it because I joined and completed the Dark Brotherhood. Do any of you have had this problem, or any other bugged quests?
Under a spoiler tag, could you go into more detail, because I very much doubt this. *spoiler* My reasoning being when you kill Nilsine and then the mother ,Tova , commits suicide that avenue is lost to get the key, but the master lock could still be picked. Obviously it would conflict with stages of the quest if you bought Hjerim before the completion of Blood on the Ice. Therefore the house should not be bought until the completion of said quest.
And in order to buy Hjerim you either have to be pretty far in (stormcloak) or complete (imperial) the Civil War quest line, which make this place one of the more laborious and time consuming houses to get. Has he completed the prerequisites to get the house?
Or maybe it's an entirely different bug he's encountering. He sure wasn't very descriptive.
If you're interested in owning Hjerim, you should read the discussion page for Blood on the Ice over at UESP. There's a very clear walkthrough for how to ensure that the house is bug-free when you are ready to buy it. You need to be aware of the steps early on in your playthrough though, so it probably won't help you at this point on your current character.