I record off SD series DGiCos all the time. You will of course need a way of recording MADI to start with. DiGiCo make a little 48 channel MADI to USB adapter - maybe whoever has the console has one of those? Other wise, RME, DirectOut and others make either PCIe card or external firewire/usb 'sound cards' to get into your computer. If you want a solution that doesn't involve a computer, the JoeCo recorder mentioned above works great.
In the console, first work out if the house sound guy/gal is sourcing the inputs you require from the sockets on the back panel, a remote stage rack, or both. If all the inputs are coming from EITHER the back panel or a remote rack connected to MADI 1, there is a simple method of 'copy to MADI' on the Audio I/O page that enables you to easily copy all the incoming audio from one or other of those sources to the MADI 2 port on the desk. This would feed your recorder.
If there are signals on BOTH a remote rack and back panel inputs, you will need to set the source with the most channels to copy to the recorder - lets say that's the stage rack, and individually route the others - back panel inputs - from the insert out points on the channels to the same MADI 2 port your recorder is on, choosing channels not used by the stage rack. Channels on the record MADI 2 output that are individually routed from the channel strips take precedence over that channel being copied from MADI 1.
The tracks you record will be after the Head Amp settings, of course, but before the live console's eq, dynamics, fader etc. In fact many FOH engineers return the recorder playback to the MADI 2 input, and a single 'listen to copied' switch allows them to do a virtual soundcheck with all the inputs sourced from playback in the band's absence.