I think this is an argument with neither side been "right".
The groundsmen are obviously at fault to a large degree, as a supervisor you should be aware of your staffs workload (and not allow them to disappear for a couple of hours to do other "duties "). He should also know how many staff he has on a daily basis (4 or 5 divot men???? Surely it must be logged somewhere ). He has obviously failed to supervise correctly.
As for the assistant he clearly abused his position by doing the kit in groundsman hours, cctv and card use prove that, as does his inconsistent account.
The state of the pitch, two managers complaining plus an independent report show their work was below standard.
That said, the management need to take a large chunk of the blame too, why was the groundsmen hours not logged to accurately record toil hours? Why was the supervisor staff list not monitored (surely you want to know who is working for you? ), and was a structured disaplinary process in place (ie oral/written warnings issued, action plan for pitch quality agreed??)
Long and short of it, both sides look ridiculous