I have lost my phone and got a new one. Unfortunately I did not backup my WhatsApp. There is a chat session held between me and someone else which is very dear to me. I have this person's phone. I wonder if I can restore this chat on my phone through that other person's phone. Thanks.
The easiest would be to use the 'email chat' option. This would send you a copy of the conversation to your email.
Open the chat on your friends phone, Press on the menu on the top right>more>email chat. Put in your email address, and you will have a copy of the conversation.
There is no way to port the conversation over to WhatsApp and make it appear as if you never lost it. Unless you have a backup.