Knackered Grapes

Hello all,
             A friend has asked me if any of you could sort out this problem for him. He has a Hamburg grape vine which is about 20 feet long and well established but it always suffers from what looks like powdery milldew. in the past he's tried a mixture of semi skimmed milk and water but this hasn't worked. I looked on the internet for him and if I'm right he should use sulphur dust in the early stages. The thing is I'm not sure that I've diagnosed it correctly and I don't want to be responsible for him losing his crop next year.
    I took a photo of a bunch of the infected grapes and posted it on My Images(hopefully!). We would be grateful if any of you could give some advice on what it is and any products or measures that could be taken to prevent it in the future.

         Thanks!