Mate tried do jump his passat when it wouldn't start, thought it was flat battery, put a booster pack on it and something strange happened with dash lights all dimming, lights shone up again as normal when booster pack disconnected; but then wouldn't even try turn over, ignition comes on but nothing happens when hold the key in to start, it doesn't even click.

It started with a bump start and drove ok, but as soon as turned it off it wouldn't start again. Just ignition lights come up, no click or anything.

All the wires / earths from the battery down to the starter and fuse links etc.. all look ok and cleaned them anyway.
All the fuses in the cabin and in the engine bay are ok.

What's the best thing to start with, gonna check for power to the starter motor I suppose.

Can VCDS be used to help with this issue?