had similar problem with my immobiliser on both the 968 and on the previous 944. with both i found that rubbing the tines of the immobiliser key with my tie sometimes helped it to trigger - but only with silk ties!
alternatively he could try using the spare key (if he has another set), a change is as good as a rest.
also check he hasnt got any damp in the floorpan, where the immobiliser unit is housed - over time they can get twitchy if water gets anywhere near them.
commonly however the female receptacle behind the steering wheel is the problem. over the years the plastic shroud into which you push the key can get worn, loose or break, so the key tines dont make proper electrical contact. ultimately i fixed the problem by changing the immobiliser key receptacle, which is a part you can get without needing to muck up the whole system. I got one from Hamilton & palmer - http://www.hamilton-palmer.co.uk/ - if memory serves it was only about £20. took me a few hours to do because you have to isolate the immobiliser cord in the driver side footwell, which has at least 6 colour coded wires, and match and connect to the new cord. however, it was straighforward in the end, and i am an utter numpty when it comes to electrical stuff
