Like some other posters I struggle with the idea that a 13 year old who is 5ft and weights less than 6 stone can be mistaken for a 17 year old. However, the jury saw her and decided it was reasonable for Richard to believe she was 16 or over. We are not going to see a photo of her so I have to accept the jury's view that she could indeed be mistaken for 17 year old.
He was also charged with paying for sexual services of a child. Believing she was 17 is not a defence to this charge so presumably the jury decided that the prosecution had not proved that the payments were for sex beyond reasonable doubt. I don't know but the fact he paid the same amount to her friend with whom he did not have sex may have played a part in the jury's decision.
It was down to him to make sure he wasn't having sex with a 13 year old. Just like it is someone's responsibility to make sure that the person they are having sex with isn't highly intoxicated.
As the law stands neither of these statements is true. They may be morally true but legally they are not.
If she had been under 13 his belief as to her age would have been irrelevant. For a girl aged 13 or over the defendant can argue that he reasonably believed the girl was aged 16 or over. There is no requirement in law for him to have validated that belief.
Similarly there is no requirement to breathalyse a potential partner before having sex. The question in law is whether the defendant reasonably believed that the victim had "freedom and capacity" to consent. Juries do not generally expect defendants to have checked. They do, however, expect them to spot if the victim is clearly so intoxicated that no-one could reasonably believe they had capacity to consent.