So, this would explain part of the issue, but not all:
8 August: car was registered to A N Other;
8 August: car purchased from A N Other by your friend;
Contravention date: 14 August;
V5C not amended until: 15 September;
First PCN sent to A N Other who makes successful reps that they were not the keeper by virtue of having sold the vehicle before the contravention;
Fresh PCN issued to your friend, almost certainly in time.
Your friend was the owner prior to 14th, but they're adamant 'that on that day (evening) he was making last preparations for holidays next day and there is no way he was in the area.'
So, who did have access to the car, was it even in your friend's possession on 14th or perhaps lent to someone else/in a garage?
IMO, simply saying it wasn't him won't succeed because he admits to being having bought the vehicle and therefore presumed to be the keeper on 14th.
By the way, IMO paying £15000 for a car valued at £8600 is usurious, but there we are.