I think the swiadek needs to have all 'papers' in order (christening, Communion records) and of course go to confession.
I think it depends on the priest who marries the couple. My brother and his future bride had a hard time convincing their priest that her sister could be a Świadek at their wedding, because she didn't go through the Confirmation. He finally gave the a green light when she started attending Confirmation classes.
But when my cousin was getting married and she was worried that the priest would make problems beacuse of her "Swiadek's" pregnancy (she was not married), the priest told the that the role of "Swiadek" at the wedding is to be a witness, that the two people who are getting married, vow their commitment, and for that purpose it isn't even neccessary to be a Catholic or Christian.
Now, with the Godparents the rules are far stricter, but that's a different story :@)