It's like telling a person having a seizure, just relax man. Or a person with Tourette's syndrome: "try not to say that word man, it doesn't make sense". Normal people stutter when they're anxious or hyped up. But that doesn't mean that stutterers are always anxious or hyped up.
It's easier for people to arrive to a conclusion (possibly an invalid one) about a man's suffering, than actually admitting they don't know what the cause is and try to solve the problem interactively with that person. e.g. She has lupus, oh it must be because of her weird diet. etc.
In your case, the best thing you can do is listen. Try to ask them questions like: have you noticed any pattern that decreases the frequency of your stutter? how about you try this or that? You also have to be careful and try not to be pushy about it. As a stutterer myself, I came to a point where I just got used to the social awkwardness and starting to accept to generally accept my stutter. It's giving me way less anxiety than it used to do. So, I'd be a bit annoyed if someone whom I don't really know that well try to interactively solve this "problem" with me. Because it's only a problem if it's causing me discomfort.
Also, showing facial signs of petty or discomfort is the worst thing you can do. When you're listening to a stutterer, don't break eye contact, because it shows that you're uncomfortable being in their shoe. treat it as if it's someone saying "ahh" "emm" a lot, most people filter out these noises unconsciously, try to do that. I understand it's a lot to ask.
Imo, the absolute worst thing you can do, is to talk to them a lot about it while conveying that it's a "problem" that needs to be solved and always trying to find a cure. My dad used to be very pushy about it and it caused me some serious psychological harm, because I never felt that I'm good enough, and I'd argue that it caused me to stutter even more, because he always wanted me to be perfect. Instead of discussing their weakness, try to talk to them about how they can leverage their strengths to be the best version of themselves.
Hope that helps, feel free to message me if you have questions.