The person might of literally disconnected offline. If the person was a passerby to aquantance than you wouldn't of gotton that option. If it was originally an aquantance than the message should apear. Oh and there is
one more if you asked to see the persons pr video if the person is already an aquantice, you will not get the option to add this person to your friends list. If you battle or trade with them than you will get the option.
I hoped this helped!!!