I suspect my friend may be in an abusive relationship?

All you can do is be her Friend,make sure she knows if she needs to talk you are there to listen,that;s what friends are for,in good times and bad times. Do not keep reiterating negative things to her about this guy either the way you are doing,as that plants seeds in her mind,then she reacts this to him when they are together,which if he is abusing her,could get her hurt,and indirectly,you would have caused that. Be a friend as I said above,so she knows when she is ready to talk,there is some one she can turn to. There will come a point she will if he is hitting her,believe me. And unless it becomes very clear,that this is happening,there is nothing you can do to help,til she herself calls for it. Just quietly keep an eye on the situation,and don't perpetuate any more trouble into it as I said,unless she asks for your thoughts concerning him.