Oh, thank you all so much for the advice and for sharing your own stories

I think I will speak to the boy's mum, as she should know what her son has been doing and yes, hopefully that will put a stop to his behaviour.
I will need to speak to DD, but will have to calm down a bit first, and choose my timing (she's really very bad at emotional discussions or having to face up to consequences). But I will tell her I have read her texts. She will be horrified. But that's my line of approach - that once it's written and sent, you have no control over it any more. Thoughts?
I have looked at her Instagram. I can't see any photos (I'm a bit shit at tech and I think she may have deleted things as she goes) but he was definitely asking for a photo of her tits at a ridiculous hour of the night last night 
I have taken her phone and will hold onto it until this is sorted out. I will show his mum the messages as I think she needs to see her son calling my daughter his little slut/whore and asking for pics.
I am bricking it at this as I really hate confrontation. But you have all helped me to see that I need to be the adult in this situation and face up to my responsibility. I don't want to involve the school or the police at this stage but I think I may threaten that if there are any future incidents.
I want her to have a nice bf, I don't mind a bit of snogging and snuggling, but this is just nasty and I want it, and her, to stop. I can hardly look at her today.