That is concerning. I’m a stutter all my life, also introverted. I would also give up saying things. One thing I hated my parents would yell at me to slow down. So don’t do that. One thing I noticed when I was about that age is I would only have a severe stutter when talking to some people. Mostly family. Does she only stutter around family or at school at well? Or in social situations? My family was great, so I wasn’t nervous around them. So if she only stutters around you that doesn’t mean a lot regarding your relationship.