How should I have acted/how can I do better in the future?

How should I have acted/how can I do better in the future? I was at work today when a customer came to the counter and tried to ask me where an item was. He could repeatedly say, "where is the h-' and then had a serious block and couldn't finish the sentence. It took me a minute to figure out what was going on as I have never met someone with such an intense stutter. I grew up with a friend who had a minor stutter and I remember them saying it was very irritating when people tried to fill in what they were going to say. So I didn't do that, and I also didn't think it would be polite to suggest pointing or writing it down. He struggled so hard, for what felt like minutes trying to get it out and never managed. He eventually said, "here, I'll just write it down so we're not here forever." My question is, did I handle this correctly/how should I have reacted differently? It felt so rude just staring at him forever. I'm sure it felt longer to him but honestly I thought it would never end. Should I have helped him somehow?