This came from one of my mum's friends. In the UK, a can of condensed milk is 1/2 pint (--300g, 10 fl oz) so the can makes a useful measure to get the right amount of water and whiskey. This recipe does work better with cheap whiskey, but there are limits - if you've got a choice in the shop between "cheap" and "ridiculously cheap" go for the cheap stuff - there is a difference! :o) Condensed milk is the thick, gloopy, sweet stuff that Winnie-the-Pooh likes with honey and bread, evaporated milk is a lot runnier, not sweetened and won't work with this recipe. In the UK, the brands are Nestles or Fussells (the one which used to have a butterfly as it's logo). If you normally drink real coffee, you can probably substitute 1 can of real coffee for 1 can of water and the teaspoon of instant coffee powder, although I haven't tried it.
Oh yes, my 2 pints is 2 1/2 US pints because we have more fluid ounces in ours!