Vow renewal

I was talking to my friend today who is renewing her vows on Sunday. She's been married 10 years. They had originally gotten married after high school in her home country by a justice of the peace. So I think this is their full Catholic wedding.

Anyways, she called to tell me when the rehearsal was on Saturday. I didn't realize that I was supposed to be there. She said that since I'm a witness I have to be there.

I did go down a few months ago and I filled out some paperwork for her stating that she had never been married before, when she was baptized and bunch of other questions. I know that was called a Witness form. I didn't realize that I would be a part of the ceremony. She never mentioned dresses or anything special I had to wear or do. Its just going to be a ceremony with a small reception with food and cake at their house after..

What does a witness do?