Check all the tyres are 'N' rated (they should ideally be of the same make)
Check the 'oddments' compartment hinge isn't broken.
Check that the VIL sticker under the bonnet matches with the Service book and contains the 'C16' (UK spec) option.
Check that if there is any smoke from the exhaust at startup (which there invariably will be with any Boxer engine layout) it doesn't last more than say 20 seconds.
Check for rust, or bubbling paint, behind the door catches.
Make sure the tool-box is still present (behind the spare tyre)
Check carpet behind seats, make sure they are not damp (usually caused by the roof drainage channels being blocked) this could cause havoc with the ECU.
Check disks, make sure they are not rusty or cracked (see if you can feel the rear of the disk, that is where they are most prone to rust)
If it's a tiptronic, make sure the lever moves freely, and in manual mode, shifts up and down the gears smoothly.
Make sure all the switches work, including the electirc seats.
Look out for 'Lux Pack' which includes a wind deflector,top tinted window, larger battery, full leather and 17" alloys....these are very desireable options.
Listen out for a knocking sound while test driving, the suspension bushes may need replacing.
A rumbling sound (especially round corners) may point to a wheel bearing going.
If all of those are ok i would then get a Porsche 111 point inspection done on it. (about £200) Then he can get the Porsche Extended warranty for £725 (comes with Porsce Assist Europe Wide) once any faults are rectified (can be done elsewhere)
Good luck!