she is 15. it is illegal to have sex with someone under 16, but it is not 'rape' that he would be charged with, it will be something like 'sexual activity with a minor under 16' or something like that (not sure of the wording). If she was under 13 it would be a charge of 'rape' whether or not she 'consented' as rightly said, a child cannot consent to sex.
If a 15 year old has sex with someone of a similar age, despite this still be illegal, they are unlikely to be prosecuted, depending on the age gap, so for example a 15 and 16yo having sex would unlikely attract prosecution. A 15yo and a 20yo is more likely to, depending on a multitude of facts, and, as is in this case, an adult twice the age of a 15yo would almost certainly be charged with the offence and when found guilty placed on the Sex Offender Register for a period of time (but not necessarily indefinitely).
Also, although he is a sex offender, assuming of course that he has had sex with her, he is not a paedophile, as that is a particular category of child sex offender who has sexual thoughts/acts towards non-pubescent children (under I think it is either 13 or 11). It is a distinction that needs to be made, as despite all the moral issues around what is happening here, in some cultures and societies it is not illegal or unacceptable to have sex with a child aged 15 and younger but in pretty much all societies and cultures, paedophillia is not acceptable.
That is not to say this is not a worrying situation in terms of breach of trust and grooming of a young girl.