I try to keep her occupied with sports etc but if he doesn't text her, she gets very upset and goes into a terrible mood.
There is a problem here and your DH needs to take his head out of the sand and start taking an active role in his daughter's life.
Take her phone away after 6 pm.
Make your DH find some activity she and he can do together once a week. It doesn't have to be a massive, time consuming hobby. They could go shopping or have a pizza or go to the cinema once in a while.
Or he could just take an interest in her homework or her female friendships, and so could you.
It is really, really important for girls to have girl friends, and this exclusive BF relationship will get in the way of that. She will have nobody when this boy breaks it off if you allow her to neglect the forming of female friendships.
Find an activity for your DD to do. Martial arts appeal to some, then there is dance, drama, gymnastics, swimming, diving, music, tennis football - etc. DH can show an interest in whatever activity it is.
@BillHadersNewWife and @BeesKnees4 - well said.
Couldn't agree more.
At first I found it cute as she never really had any boys interested in her at primary school and vice versa, so I thought I would let her enjoy the attention
The DH may have something of a point when he blames you for encouraging this, somewhere deep down.
But he (and you) need to sit DD down and tell her the intense focus on what this boy feels about her is all wrong.