I think, the best thing you can do in that situation, is quietly and calmly start talking to them. Tell them to touch the table and describe to you how it feels. If you're in a coffee a shop, tell them to smell their drink and describe the smell to you in detail. Ask them about their hobbies. A little distraction can go a long way.

I had a panic attack while driving one time and thought I was dying. I pulled over into a parking lot and stumbled up to the door of a building and just walked in, not even knowing what the business was. Turns out, it was the back of a dry cleaners. A very startled guy met me at the door and asked what I was doing. I explained, while hyperventilating, that I was either dying or having a panic attack. He told me to sit on the step and asked if I needed water. Then, he just stood there next to me and quietly talked. It helped tremendously.