How to tell sudo to repeat lecture once

The shell's parent process is su -, and the parent of that is the sudo su -. So you need to find out the user running sudo su -'s parent process by searching back in two steps with ps:

ps -o user= $(ps -o ppid= $(ps -o ppid= $PPID))

But you shouldn't be doing sudo su - if your version of sudo is not too old to have sudo -i. Sudo sets the environment variable SUDO_USER to the name of the user who ran sudo. You won't see it with sudo su - because su - scrubs the environment.

$ sudo -i
# echo $SUDO_USER
gilles

See man sudoers

lecture_file
Path to a file containing an alternate sudo lecture that will be used in place of the standard lecture if the named file exists. By default, sudo uses a built-in lecture.

So, if you wish to change the text, add it to a file and point to it from /etc/sudoers:

Defaults        lecture_file = /etc/sudoers.lecture

According to the change log for Ubuntu Qantal, sudo is now compiled --without-lecture.

The default text, when compiled in, is:

#define DEFAULT_LECTURE "\n" \
"We trust you have received the usual lecture from the local System\n" \
"Administrator. It usually boils down to these three things:\n\n" \
" #1) Respect the privacy of others.\n" \
" #2) Think before you type.\n" \
" #3) With great power comes great responsibility.\n\n"