But DSD15 is just being awful to him. She's told him she's stubborn and he thought he could win her back with sorry cards and flowers but she hates him and never wants to see him again.
No, she's not being awful to him - she's behaving like a perfectly normal 15 year old who has not been given appropriate boundaries and as a result beleives she can manipulate her Dad to get what she demands.
A parent shouldn't try to "win back" their DC - it leaves them feeling insecure and overwhelmed by the responsibility that gives them over their parents emotions.
The fact that she is still seeking financial handouts is actually a good sign - it means that despite her behaviour and his response, she stills see him in a parental role at the moment - if he shapes up and starts parenting her, then the chances are this will all blow over.
I think the wording of the text you suggest is spot on; and if she chooses not to reply, or contact her Dad in another way, then he can continue to remind her he loves her in other ways - a postcard now and again telling her what he is up to, a good luck text when she has an exam or similar coming up; just keep the lines of communication open and more importantly, ensure that she KNOWS him, so he's not a stranger if she ever does need him.
I understand that he is digging his heels in; its so easy to do - but perhaps my experience will shock him into changing his mind. My DSD chose not to have contact with her Dad for 2 years after some perceived slight; she gave him an ultimatum - me or her. DP told her there was room in his life for both of us, and DSD Mum supported her to stay away. DP kept in touch in any way he could - and ignored her Mum who was telling him to stop writing to her, not to go to performances she was in, and not to keep in touch with her school because DSD didn't like it.
When things went wrong between DSD and her Mum, DSD didn't hestitate - she called her Dad for help.
If your DP refuses to be a parent now, then his DD may not have a Dad she can turn to when she needs one most.