The system messages for the email form should include the target/recipient username as a parameter
e.g. the first form on English Wikipedia already parses the username from the page title
But this isnt possible for the landing page after the email has been sent.
Then the Emailsenttext default message could change from
"Your e-mail message has been sent."
to something like
"Your e-mail message to [[user:$1|$1]] has been sent."
fwiw, the messages used for the email do include usernames