as a white middle class liberal, I am not in a position to say to a black person who has had the word used against them, 'you are not to use or not to reclaim this word, it is offensive.
Of course you are.
You think it an abhorrent word.
To choose not to point that out to someone based on the colour of their skin is, in my opinion, racist. You remaking a decision based on the colour you see.
Now, if you spent time talking to that person about their use of the word, your liberal dilemma, your respective experiences etc, then you'd be in a position to judge.
But blanket decision based on skin colour? When it happens to me, I feel patronised. I don't want to be spoken for or have racist words okayed by someone who is freaking about about whether they appear small-minded rather than just bloody well asking me how I feel.
I think everyone is able to decide for themselves.
I'm not white. I detest the word. Please don't speak for me or excuse it in order to establish your liberal credentials. 
This is why I love the British