His only failing would be not to cover it up properly. Mixer wash out has to be done in a timely fashion or it can be rendered unusable. The gravel is very small for that type of work and to bury it in a corner of the yard in common. The Portland is washed out of the gravel and doesn't solidify into one giant chunk of concrete.if it is covered properly you would never know it was below the surface. When the house was built any residual concrete was treated the same way. The main thing that shouldn't be buried would be termite food...any wooden scraps.
It is not legal for a qualified contractor to just dig holes anyhow and then later on leave litter in someone's compound. That is a very bad behavior and he can be sued by the local authority for littering around.