I read your post about blessing your friend's house... I have to say, it's tricky. I agree partially with the apologetics answer, but if I were in your position, I would still think and pray hard to see if that really is the right thing to do. Especially pray for your friend.
Perhaps telling your friend that you feel really uneasy being in the house knowing about the witchcraft and if it would be okay if your friend "humours your superstition" and allows you to bless his/her house for your peace of heart? Most of my friends would laugh but agree if I said this really seriously and explained that I was genuinely concerned. Especially if you add: "It makes no difference to you, but it would make me VERY happy."
Hope it works out! God bless!