I would hope that by their third event I would have introduced myself 
and would therefore know them and they would know me. I do believe that
one of the most important functions of a baron is to make new people
welcome. I think I can be diplomatic enough to tell this person that we
are delighted s/he has returned to join us again, but that our Society
does have some minimum guidelines about attempting to wear clothing from
our time period. We have many solutions the problem: our baronial Gold
Key is ready with loaner garb for the short term, and there are many
people who would be willing to help that person put together some
clothing even if they have little money or skill. How I would behave
after that point is a function of how they react. If they are sincere
and willing to play, but hampered by poverty or other constraints, I
would try hard to find them cheap solutions, including passing on old
cast-off garments. If they aren't particularly cooperative, then I would
be less inclined to go far out of my way to help. If they were
uncooperative and showed up again the same way, I would have to be
sterner.
