Kant gave us the example of an axe wielding man, he claimed that should a man carrying an axe knock on your door demanding to know the location of your friend, you are obliged to tell him only the truth.
The reason Kant gives for this, is that it is our moral duty is to tell the truth. It is a sub-set of the Categorical Imperative, that we ought to do ‘x’ – in this case tell the truth – as lying would fail to be universalizable, it wouldn’t allow the world to function if everyone lied – there would be an excessive amount of mistrust and false information.
So therefore Kant argued that we should tell the ‘axe man’ exactly where our friend is. As for whether or not your friends potential death is on your conscious, Kant said that the only reason you should feel the guilt on your conscious is if you lied and the axe man killed your friend anyway. Whereas if you told the truth and your friend died, that should in no way form any kind of guilt in you.