Previous comments pretty much echo what I'd say. For some stutters it's that beginning part of the conversation that gives them so much trouble. Once they're past that, things tend to even out a bit. It can also vary day by day, with some days being more fluent and others less so. If someone's having a particularly hard time getting a word out, I'd think noticing their non-verbal communication is just fine, or perhaps asking a question to confirm their choice. It gets them past the block and frankly is a relief. But what I really wanted to say is thank you for being so conscious and respectful. Your patience and thoughtfulness is very much appreciated. It's this type of everyday situation that can be a source of anxiety for stutters and your kindness will help the person feel more at ease and welcomed.