A few questions:
If you haven't looked at the texts how do you know the coach is constantly texting her at night?
She is a 15 year old girl, I'm sure other teenagers are also texting her a lot too.
It may be something as innocent as "Hey good run today, it's really going to help this next season..." and a reply or two after that. But if it's a lot more than that I'd agree it's probably too much. But if that is all the coach is doing and someone else is texting constantly, don't take it out on the coach.
I also disagree that you should go to the AD if you think it's innocent but too much. Any time you go to the AD before going to the coach it's a hassle for everyone and makes the coach look bad even if that isn't your intention. If you think the texting is innocent but just too much, have a conversation with the coach, talk about why he thinks it's necessary to text that much, ask if there is different ways he could get his information across. How that conversation goes and what happens after will tell you if you need to talk to AD or anyone else.
As a HS coach I text the kids a lot, it's just a different day than it was 10 years ago, that's how you get a hold of them, that is how you can build a coach/athlete relationship. Kids will sometimes open up more in a text than they will in person. But as a coach you also have to be smart and cover yourself, I use group texts a ton and half the time am communicating with the kids at parents requests.