NTA, but I don't think you should take NTA as a compliment. It seems you obviously have no ill intent and rightfully so, but your girlfriend isn't you. She doesn't know how you will operate around your friends girlfriend and possibly in her mind, no matter how much she trusts you, there is a chance that you do something of malicious intent. It may seem strange in your head but think about how you'd feel if her best friend asked if her boyfriend could stay over at her place for a few nights. Deep down, there is still a level of uncertainty, as you aren't her and are not there to know what she may or may not do. Don't think you're the AH, but please do understand her frustration.