I designed a PCB in Fritzing, but forgot to mirror it. How do I fix it?
If I understand your problem correctly, you do not need to mirror your copper layer.

Consider a "real" layout with a "real EDA" software like Eagle or PADS. The layers have to have the same coordinate system and 

The output with coordinates are always orientated as if you are looking onto the populated side of your PCB. Otherwise it would be rather difficult to align the layers correctly. 

So the bottom layer has to be mirrored by the manufacturer anyway. Because of this I assume your guys at the hackerspace just can take the top layer, declare it as a bottom layer and it will be mirrored by the CAM software correctly.

For viewing the Gerber files to identify all layers correctly I recommend the free tool GCPreview.