This depends on the exact situation. Assuming I know
it is for the third time, that the person is not
immediately leaving the event, and, to be quite
candid, I am not in the middle of some other small
crisis at the time, here is what I would do.
<p>
If I could find someone who knows the person, and that
someone is willing to talk to the person, then I would
suggest that the someone have a chat with the person
in the very near future and possibly take them to
visit Gold Key to see if there is any loaner garb at
the event.
<p>
If no one knows this person or is willing to talk to
this person, then I would introduce myself to them and
ask about their interests in the SCA. Then I would
point out that some attempt at medieval or Renaissance
clothing is expected at an event. If he or she still
looked friendly, I would expound on the virtues of
tabards and t-tunics. Then I would tell him or her
that we would appreciate it if he or she would make
the attempt to wear some type of garb. If they still
looked friendly, I would offer to accompany them in
search of loaner garb. 
