Ask her to at least respect that you are trying to get some sleep and turn off the phone she can check the text in the morning. If she is reasonable she will respect your wishes she doesn't have to answer her texts as soon she receives it, you should also ask her why he cannot text her at a more reasonable hour maybe he lives somewhere that there is a time difference and doesn't comprehend that you may be sleep. If this is the only issue then you should be able to talk through this just tell her that it's important that you get sleep and this is your only request is for her to turn her phone off at night when she is sleep so that you don't have any interruptions.
It could very well be that he may like her more than a friend but she may just be trying to be a friend and true friends are there for their friend day or night, she might like the attention but if she were trying to hide anything from you she wouldn't let you know that he texts her or she would text him away from you. I would tell her that you are tired of losing sleep because he wants to be disrespectful and text her at night or tell her to put it on vibrate so that you won't hear it. If you are cordial with him enough to say something to him then maybe you should do that if he knows it bothers you he may not do it anymore so not to cause a fight between you and his friends (your girlfriend).