Hi Evans
My take on thisL
1. First incident "not in the course of employment" - ergo would not have been RIDDOR reportable even if the injury had met one of the thresholds.
Second incident - probably "in the course of their employment".but subject to argument that the work element might have finished and now, effectively in "own time" - or as one judgment in the case law says "on a frolic of their own".
Now many of those who might agree that the victim was in the "course of their employment" would go on to follow the HSE guidance on RIDDOR and conclude that it was NOT reportable despite meeting a RIDDOR injury threshold - broken bone in foot other than in a toe.
However, based on the actual wording of the Regulations rather than the HSE guidance which has always seemed designed to discourage reporting of reportable accidents that are not going to be of any interest to the HSE in terms of follow up - rather than the primary purpose of RIDDOER - to provide intelligence about numbers of accidents and their immediate causes.....in my book reportable.
2. Of course the victim can sue, but who?
Employer has duty of care but this is dependent on how much control they had over the condition of the premises where the accident happened and if off site, probably none, and likely that any case against the employer would fall IF put to the challenge and not quickly settled out of Court.
Now if the tree is an ent [cf Lord of the Rings] with attitude and lifts its roots to exacerbate the tripping risk then possibly a case against the tree but there is no point in suing somebody with no means to pay compensation, unless doing only for the verdict and not the pay out.
If the tree is on land owned by somebody other than the employer [or even if the land IS owned by the employer] we are into Occupier's Liability with slightly different rules in e.g. Scotland than in e.g. England & Wales.
However there were a few judgments in the early 20th Century which established that in general there is no duty under Occupier's Liability to protect against injury involving a natural hazard that is obvious. A case involving East of Scotland Water Authority much later confirmed that this principle applied also to artifical structures posing very similar risks to natural features of the environment - so the drowning risk in an above ground reservoir v any lake.
All these judgments then being supported in the 21st Century in Tomlinson v Congleton Borough Council but with the caveat that the exception from liability might not apply if there was something UNUSUAL such as an adverse camber on a path.
Various guidance on all this, e.g. from RoSPA, Visitor Safety in the Countryside Group and Environment Agency.