Not in the right area to help with the fitting, but knowing which nav system he has bought would help, is it an RNS510 for example. If it is then there are hacked firmwares to get you around the battery drain issues with the can gateway.
Also I believe in the Leon he will need a new piece of trim to fit the nav unit as the original stereos aren't the same shape. So he will need to look into that as well.