Your friend parked inside a permit parking zone where the restriction is announced to motorists at the entrance(s) to the zone. Inside the zone no signs or lines are required, although if the zone is large, then repeater signs are recommended. In this case, the zone is only a single street, and your friend parked about 126 metres down the street from the entry signs.
The signs (one per side of the entry street), restricts parking to permit holders only, from Monday to Saturday 8.00am to 6.30pm. Presumably he would have passed these signs on his way to the parking location. So what made him think it was OK to park there without a permit on a Saturday within the restricted times ?
Although we normally recommend submitting representations against a PCN, I really don't see what the content would be in your friends case. The only thing I can see is that the sign on the left as one enters the street is not totally facing any approaching vehicle. However, there are two signs, so I don't think this is a powerful enough argument to convince an adjudicator, (BTW the council will refuse all reps). Maybe others on here can suggest some more robust arguments, but I can't.
Addendum
Can you please tell us more about this: -
He was going to work
Where does he normally park ?
For info, if work is as a care worker or similar, and the person being visited lives on the street concerned, it is normal practice for the resident to issue a Visitor Permit else nobody could visit residents in the restricted days and hours.