Hi, could anyone suggest proxies or sources for building a 6mm Danish Army for 1864 please. Got too many projects on the go at the moment so asking for a friend ????

Union infantry in kepi are a reasonably good sub for Danish infantry. If wanted with a greatcoat you could try 1866 Austrians with a filed down shako to make a kepi. The French in greatcoat are carrying too much equipment for Danes.

The Italian Risorgimento range is also useful:

Piedmont artillery are in kepi, Piedmont infantry with filed down shako could also be used for Danes in greatcoat. Austrian dragoons from the same range can be painted as Danish dragoons.

Napoleonic Austrian hussars NAU8 could be painted as Danish hussars. 

I would also recommend a search of the Napoleonic cavalry as there may be others your friend may consider more useful.

My Danes on parade here. Infantry = Baccus ACW Union inf as recommended by Andy above. The rest are Heroics & Ros: Hussars = ACW Union cavalry; artillery = ACW Union arty; generals = spare ACW mounted officers with putty conversion headgear; dragoons, not sure, something Napoleonic (maybe Austrian dragoons as per Andy's suggestion).

Note that the basic line infantry and dark blue with light blue trousers serve as generic line infantry for my Greek 1897 army as well (and indeed for Belgian C19), not to mention as actual Union troops. I made a load of standardbearers from spare gunners. Similarly the gunners in blue can do duty for several C19 armies. One virtue of 6mm is that you can get away with such things!