The dress code will depend on how religious the family is. Religious people would expect a woman to wear a long dress covering her knees (even when she is sitting down) and elbows.
Selecting the appropriate present is a more complex question. While money may be OK, especially for a secular family, Jewish tradition looks at this occasion as an important religious event rather then an opportunity to demonstrate splendor. The traditional gifts are Jewish books. You may also consider giving any other book that is about personal responsibility – the main theme of bar mitzvah.
Sasha
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Work is a matter of taste. If you don't work you don't taste.
