This is the crux of the matter, you can moralise all you like, but law is the only thing that counts.
What he is doing is not illegal, nor is what she is doing. She is offering a service which he is subscribing to.
I have an ex pupil and now registered gas fitter of 24 years old service my boiler, it's the same thing, from a legal stand point.
In the case of this teacher and only fans girl, the power imbalance that existed when he was her teacher has long since ceased to exist, you could argue as her customer the balance has shifted.
Some schools do have policies on how long after a pupil leaves you can interact with them socially, but these would be legally unenforceable, the professional standards don't set any timeframe after a pupil leaves at 18, because they can't, legally.
Schools own contact recommendations exist to protect teachers entering into relationships with ex pupils from allegations they started said relationship before the pupil left. They don't cover buying a pupils service, be that boiler servicing, a coffee from their pop up coffee shop, lift in their taxi, or indeed only fans content.
My personal view is that it sounds very seedy from both parties, but my view that OF is seedy doesn't count, it's not illegal.
You would have to prove that something was actively in play before she left school to have any genuine complaint.
If however she discloses details to the school of her customers, she breaches GDPR and he would have a good case against her.
But is she bothered?, she's getting paid for the service she has chosen to deliver.