yes. These movements are called accessory behaviors. They are fairly common, although head shaking and flailing arms is on the severe side. Regarding what you can do: (1)Be supportive. These are not intentional movements. (although they are not strictly involuntary either - they're in between). (2)Ask her what she would like you to do when you think she is struggling. Not everyone is the same or has the same reaction to stuttering. I do think your suggestion she sees an SLP is a good one. But, you're right, that's her choice. Some lower "risk" options could be a stuttering support group, like regional chapters of the National Stuttering Association.