A side note though, a fare evader is a passenger but, this time, not a customer.
So, at the risk of opening a potential can of worms…
A person who boards the train without a ticket with the intent of not paying is an (illicit) passenger but not a legitimate customer of the railway - that's fine, I doubt anyone can dispute that.
What about a person who has no ticket because they thought they could pay on the train? Someone who was confused and bought the wrong ticket from the machine, or was sold the wrong ticket by a clerk? Someone who forgot to collect their ToD tickets, or didn't realise that the confirmation email
wasn't their ticket? Someone who didn't realise their reservation was on another coupon and left it in the machine, thus invalidating their Advance fare? Someone who didn't realise that a Senior Railcard wasn't the same thing as their OAP's bus pass? Someone who unknowingly bought a child ticket for their 16-year-old offspring? Someone who travelled to "London" Luton airport on their Oyster card, not realising that 'London Luton' is the world's most concise lie?
I propose that all the aforementioned people are "customers" (and in fact all customers who have been let down by the railway in various ways.) Therefore they should be treated as such. We need to be careful about saying who is and isn't a "customer," and preferably err on the side of someone being a legitimate customer (who deserves the respect, deference and good customer service that confers) if there is any doubt at all.
This, after all, is why BR started using the word "customer" in the first place.
(This is partly why I dislike the idea of "penalty" fares: if someone is a legitimate customer, should they ever have to pay a penalty?)