I am a legal expert in these sorts of things and I can assure you unless evidence comes from her, he would not even get a ticking off by the police, let alone a caution.
The age to be able to give consent in the UK is 13. The actual age for consent is 16.
Pedos, in its true definition is an interest in pre-pubescent kids, so no he is not a pedo.
You can't stop them, and as she is consenting (whether pressurised by him or not) there is nothing you can do)
The law is an ass, he is clearly breaking it, but as with all crimes it has to be proved beyond all reasonable doubt. without her testimony, there is no case.
Finally, there is no way she will end up in trouble, apart from up the duff if she is not careful. She can not aid and abet any offence, because you can not aid and abet a law that is there to protect her in the first place.
The only way you can really catch him, without her testimony, is if he takes any photos of the adult variety on his phone or camera. These do not have to be naked ones, but must be sexual. The age for sexually gratifying photos is 18, unless the couple are married in which case it is 16.