If you want to know how to react when someone stutters here is the best advice we can give you...
Do not say things like:
What’s the Pic Articulation is a revolutionary app for speech therapy!
It can make them feel bad, ashamed, or embarrassed which actually makes stuttering worse.
Listen to “WHAT” the person says and not “HOW” they say it.
Show you are listening by:
Wait for them to finish talking.
Do not finish sentences or say words for people who stutter.
You can make them feel more relaxed which can lessen stuttering by speaking a little slower/calmer, but make sure that it's not so slow that it sounds weird.
If you are talking on the phone, remember it is usually harder for someone who stutters so give them extra time and don’t hang up if they are trying to start talking.
The above follows advice from the Stuttering Foundation of America.