my friends lost his coins from offline wallet when he was chatting online on facebook messenger
sure he can't restore his coin
so how can protect offline wallet ?
First of all, please be more specific on how did he lose those Bitcoin.
As per my understanding there can be only two reason, either he has shared his privately accidentally or has send those Bitcoin accidentally on Facebook.
Never share the private key cause it is meant to be private and never talk about Bitcoin to strangers.
Follow the above rule and be safe.