Based on the photo, the clean section from 11 o'clock around to 2 o'clock seems to have failed recently and the crystalline appearance of the fracture surface suggests that the plastic was in poor condition and brittle, probably due to age and/or poor quality manufacture, i.e. 'wear and tear'. The section from 2 o'clock around to 11 o'clock as been broken for a considerable (months/years) length of time, long enough for the brown stain (oxides or other particles in the supply) to build up.
I'd agree that the clean section looks as though it has 'snapped off' recently but it could well have been the water pressure that applied the force that was the coup de grace.
So, in my opinion, the diverter has been in the process of failing since well before your plumber was in the roof. A visual inspection of it in operation would probably have shown it was spraying through the 'brown crack'.
In good condition, those things are pretty tough and I doubt you could break one in the way illustrated by accident. Unless you instructed the plumber to check the condition of the float valve as part of the job I don't think you can hold them responsible. If you want to recover for the cost of repairs due to water damage I think you'll need to make your own insurance claim.