Sorry to sound daft but is there anyway of checking this, to confirm that this may be the problem thanks
ive had both of these issues last year (alex's top mount bearing issue, and yours sons cv joint).
top mount bearing issue was singular very loud 'DONK' noise which echos through the front of the car when turning full lock to one side. doesnt matter if youre moving or not. caused by the spring being twisted then jumping on the because there isnt a bearing to allow it to freely rotate with the steering. (btw alex, my failed bearing was also accompanied by a snapped spring. where did you get a pair of oem top mounts and bearings for £40 out of interest)
your sons issue will only present itself when moving at full lock opposite to the failed side. so full lock right=left side outer cv joint, and vice versa. will not do it when stationary, as the cv wont be turning.
options- replace the outer cv joint, or replace the entire axle/driveshaft. i went with the latter as it was the best option for me.
just checked the parts available for your car, and the prices arent that bad (relative to what was available for mine).
only brands id use are f.a.g, skf, spidan, lobro. last 2 are owned by gkn who usually make driveshafts for vw.
if you can get an oem from a dealer, thatd be best but price wise i was quoted £150 for a cv joint alone.
price wise for a cv joint you are looking around the £40-50 mark for one from the above companies.
for an entire driveshaft youre looking at £90 for an skf, £140 for a lobro and £190 for a spidan.
there are cheaper options such as j&r whole shafts for £45 and £15 for the cv but tbh those are hit and miss according to reviews. i wouldnt put a cheapo on my car personally.
fitting, if you can fit it yourself, youre winning. a whole driveshaft is easier to fit compared to just the cv joint.
if going to a mechanic, youll pay anywhere from £40 to £150. changing just the cv joint is more labour intensive so you might find the labour to change the joint makes the part cost saving insignificant.