Tissues, water, etc, are usually when the panic attack is coming to an end.

But it's still extremely easy for the panic to come back, at least in my case. So : be calm. Don't think it's over because the person stops screaming/crying (it might happen, it might be scary but it's ok, it's just panic and it will fade soon) or has just drank a glass of water.

If the person needs to talk, be there. Listen. It's mainly about listening and paying attention.

I usually try to understand why the panic attack occured, but that's me. Anyways, simply be there, ask if you want to do something, and listen, pay attention. Do things slowly.