I have been to China last year and have spent a great deal of time touring every where.
Just be your self and wear your western clothes...they are very familar with this as they themselves want to be western!!
There will be no expectation that you wear chinese clothes.
The big thing that will you can do is learn some basic chinese..they will be impressed you made the effort.
The chinese are very well educated, and many speak english or at least are learning it. They may ask you for assistance with this.
Never allow your self to compare with how things are done in your country..it is quite different.
I have been totally captivated with the people and there country..it is just amazing, vast, full of so much culture, history..there is no end to it. Allow yourself to soak up the experience.
Have heaps of fun..

Just be your self! Don't let dressing-up bother you.
Western clothes would be totally fine, Chinese won't wear tranditional costumes but a little formal or stylish clothes should show your respect at wedding.

there is no special request. but better not red in black.

Wish your friend happy wedding.

sorry, RED IN BLACK should be wear in black.