Thanks for bringing your question to Just Answer, and sorry for the delay in response.
Your friend and his ex split up when his daughter was about 5 years old, if I did the math correctly. This is a hard age for a child to lose a parent--even though he spent time with her and coached her team, it isn't unheard of for the child to have some attachment issues. So yes, it's unusual, but not pathological (at least judging from the information in your note).
The biggest issue, however, is whether the amount of attention you get in this relationship is sufficient to meet your needs. To expect that the behavior between the two of them will change is to set yourself up for disappointment.
When the daughter finds a boy that she's really interested in, the texts & emails should slow down. In the meantime, if you want to stay in this relationship, your best course is to ignore their communications, and focus only on what is between you and your boyfriend. This will be hard, but trying to impose your ideas of what is an appropriate amount of communication and affection between them won't work.
The best indicator of future behavior is past behavior. His guilt for leaving (the most likely explanation for his behavior) and her insecurity over him leaving her at such a young age is not likely to change, and he will likely always put his daughter first.
If what he gives you is satisfying, do your best to let all this roll off your back. If the daughter sees you as a "threat" she will step up her attention-getting behaviors. Ask him to turn off his phone during your private time together--that way, he can still communicate with her, but you don't have to witness it and the drama that follows.
If you don't think you can handle this amount of interaction between them, it may be time to re-evaluate why you are with him, and how much you are willing to put up with.
I wish you all the best in working things out with him in a way that works for both of you,