Stay with them until the end of the seizure, without any other contact.
I know it's different for everyone, but in my case I just want to say that hearing stuff like "breathe, it's gonne be ok" is more stressful than anything. It doesn't help, I can't control.
Of course other people can find it reassuring, it really depends!
Anyway, if someone you know is dealing with seizures, ask them what's appropriate to do in case one happens while you're around.
It's always better to know in advance, if that's possible!