Definitely let her finish saying what she's saying. Interrupting and finishing a stutters sentence can be extremely annoying and also depressing because I feel like I failed. Be genuine and relaxed, everything about her is normal except for the stutter, so everything that you would do, do it. You'll get use to it. It is true that stuttering can be exacerbated by stress and anxiety. But sometimes it's just bad for no reason, so don't assume something is wrong just because her stutter gets worse. I've definitely sabotaged myself several times and pushed away people because of how I assumed they felt about my stutter.