I've read the complaint and I have no idea whether the SMC have a case. The crux of the case seems to be the amount of fertilizer and other products the SMC bought. Whether that was a reasonable amount and who was responsible isn't really clear, as its not really covered in the complaint. Second, whether Darrell Cook should have been working whilst Hull FC was playing and whether he was allowed to work for them. Again that isn't clear as it may have been covered in his contract of employment.
Whether the SMC have a case depends on what the 2016 contracts of employment said. Without seeing the contracts nor the amount spent on looking after the pitch its difficult to come to a conclusion.
Finally, a lesson for all employees. If you sign a contract of employment, make sure you keep a copy and keep it safe. You never know when you'll need it.