That the council have had a call, come back and rushed to add many more signs and told the client their policy is to place multiple signs facing each way and they didn't, they are/were worried about liability.
I'd say the girlfriend and council share fault, improper signing clearly against their own policy, but that there was a cone at all should have been a dead giveaway to your girlfriend.
Had the one cone just been in front of the very next drive down the road, it would be 100% on the council.
One of the bigger issues is, ****'s buy(or nick) their own cones then decide to mark off road for their own cars or friends to park. So unless there are lots of cones and lots of proper signs it is fairly easy to believe it's just a random cone some local who thinks he owns the entire road put out.
Either way, council didn't follow their own signing policy, I would pay to get it cleaned and seek reimbursement from the council, if you want to 'do the right thing', you could offer to share liability and share the clean up costs because she is partially at fault but I would think if the council haven't followed their guidelines they'd cave pretty easily on reimbursing her.