whatsapp in 2 different device

i am using iphone 4 and purchase whatsapp with my apple id. question:-

1. i need to help to restore a friend's iphone 4 to ios 5.1.1 and download whatsapp with my apple id. since i have purchase this apps i dont have to re-purchase this apps again right? it can share to a few device am i right?

2. if i download this whatsapp to a friends iphone 4, he will be getting a fresh new whatsapp or with my current usage/history in it? he can register new account? or using my phone number registration? since its purchase with my apple id.