What happen with my message on my friend's device if I deactivate Viber My question is quite clear, what happen with my message on my friend's Viber if I deactivate it? 

Thanks
Your messages will no longer be available on your friend's device/account, but will still be seen from whatever device/account you're using, but 

NOTE: All the messages will be stored at Viber's database as a backup.

I hope this answers your question.