I doubt this is something which requires photographic evidence to prove guilt.
As such, at best it will come down to your mate saying it wasn't obstructing, and the people who reported it, and the guys who towed it saying that it was.
Who do you think the decision would go with?
Edit:
Personally, I don't believe that law enforcement should require photographic evidence to be honest - otherwise you end up that the police can't do anything without filming it to prove that they followed all the rules and regs to the letter.
If they fail to do so, the b*stard lawyers will hang them out to dry in court and yet more criminals will be walking away free. That's if the police even bother to try enforcing the law in that situation. The country is already in the sh*t because we're halfway down this road already....
Unfortunately, the alternative is that the authorities have to be taken on trust, which means the odd injustice (like your mate claims) will get through...
Can't have it both ways, I'm afraid