But 13yo girls should be innocent young girls. It is very very sad that they are not, and that they are involved in adult activities, and involved with much older partners.
I would really discourage my Dd from being involved in such a relationship.
I would speak seriously with her about the inappropriateness of the age gap and the fact that she was much younger than him, and what that means. I would set very strict limits on when she could go out, with who and where. After all is 13y IS a child, and is, and should be, under the guidance and supervision of a responsible adult - and to encourage such a relationship IMO does not go well with the role of a parent.
I would speak to the boy involved seriously about the above too. I would make it very clear that any form of sexual relationship was illegal and that he could be in serious trouble for doing anything sexual with a 13y child, and how that could affect his future - having such a record.
If they insisted it was going to happen I would also be speaking to his aprents. I would be ensuring that they knew that I was not happy about it and that they were not to be alone in his bedroom, etc and under no circumstances to be staying overnight, etc. I would expect the other parent to realise that the girl was still very much a child, especially in the eyes of the law, and to treat the relationship as such.
But I wouln't be encouragig it at all.
And yes, some relationships like this last. But IMO it doesn't make it right. A 13y should not be having grown up relationships. A 13y should be enjoying childhood with children their own age.