Macy's and Davids Bridal are both good places to find formal wear and evening attire. J.C. Penney also has formal wear that would be appropriate for a dinner party.
qipao or cheongsam
No. Not unless they're a transvestite.
I would wear something both sassy and classy :) It depends what type of dinner party it is (family, work event etc), but you can't go far wrong with dresses. Wear something midi-length or just above the knee, and nothing too revealing, in classic back or a fun print depending on the occasion.
a dress
Yes, Any one should wear blazer with jeans to a party. But, That should be perfect match and attract others.
she should wear whatever she wants to wear because she shouldn't care what other people think
parties
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what ever is casual to you i guess
Depends are you a boy or a girl ;)
if you going to the Prom after dinner you should. Prom is all about dressing up and showing off and having a good time.