Absolutely an adult can be bullied, but in the hypothetical of a trainer/client relation, I see little possibility for either safe sport or usef to support a claim of bullying of a trainer by their client.
The Tommy Serio situation isnt a perfect analogy, but Tommy was found to have used his position to take advantage of his employee/working student.
You wonât likely see the reverse, because what power does the employee have to extort with? They arent obligated to be an employee, as a client isnt obligated to train with a particular trainer, so threats or fear of a client leaving arent really a thing. It might feel wildly extorting, but you provide your client a service that they can take or leave as they will.
A client or employee are also perfectly within their rights to tell people that you are a jerk who is bad at your job, and they can say that all the time to everyone possible, presuming there is no NDA in place.
They cant say things that are demonstrably untrue and can be proven to have materially impacted your livelihood for the worse.