A warning sign can be any means by which a person is alerted to a potential hazard in a manner appropriate to the hazard that is present.
In this case, someone saying 'be carefull' is accepted as such. If it the warning sign is a hazard marker, such as cones, then they must conform to H & S standards but if the warning is verbal, then that is acceptable unless the person doing the warning is likely to be distracted.
The only time a person can actually 'report' a person for a breach of H & S is if that person is qualified and competent to do so. Other than that, all they can do is 'make a report' which is very different. Then the incident would, or maybe, or not as is often the case, be looked into by someone who is qualified and they would make recommendations only unless any injury occured.
If that advise, which has to be given specifically to the person concerned, is not then followed, then the sh*t can hit the fan.
I was an H & S rep on the railways in a previous incarnation and they were sh*t hot on such things.
Cheers
