I tend to say I'll be wishing them well and trying to do something to help. Not sure it's quite the right time to rub their faces in the fact they're begging for help from the one that supposedly caused it, or at least allowed it.
My response is simple. Pray to whom? To God? If he were omnipresent,all powerful and all merciful, why did he allow it happen to them? Why couldn't he have stopped it if he was omnipresent and all powerful? Or perhaps he allowed it to happen so we could pray to him to save their souls? The question is; if he cannot save them can he save their souls? You see how illogical the belief of his existence is?
I usually just mention that I am donating to the Red Cross/Doctors Without Borders/etc to help out. Never even acknowledge the prayer request. If they push it, then I'll say that I am more concerned with actually helping than making a show of caring.
Oh, wow. I saw your response only after I posted mine. You really are my twin, evil as you may be. Seriously, next time you are in the States, let's see if the Universe explodes upon a handshake between us.