Personally I wouldn't go for any of them.
If it is focussing on milkshakes then I think it should definitely have the word shakes or milkshakes in it.
The Dairy Works sounds like a factory, The Yard, whilst a play on a song, is irrelevant (what about Polaroid Picture?

), the other three are okay but I think you will be able to do better. Although it is quite tricky because generic type names are already in use. This might not necessarily be a problem based on location of competing businesses but I think it would be good to start with a brand that can be expanded in the future without any potential issues of conflict.
My favourite in the thread is probably Shake Shack but I'm not sure it's quite right.
What is your girlfriend's name? And does she have plans to develop in to other products? Has she any plans to make the business unique or is it pretty basic shake making?