Sorry about your friend's loss,it also happened to my close friend,within seconds his account was emptied(i have read that post and damn...that was so much Steem.)
The suggestion he had is the one i also had in mind,disable any content in comments that starts with "P5...." probably bring a mesaage before a person presses the post button.
There is already a warning when posting with strings that look like WIFs (formatted private keys). I was giving away credentials for a testnet account. Such a change would possibly only require modifying a single string.