There wouldn't be a CCJ in the sense you mean - it wouldn't affect the credit rating or anything like that.
Good info here about starting a Small Claims' procedure:
http://www.adviceguide.org.uk/index/your_right s/legal_system/small_claims.htm#Types_of_case_ dealt_with_in_the_small_cl
The court would want to know the steps your cousin has already taken to retrieve the dress. If she hasn't exhausted every reasonable option, her claim could be refused. The court is a last resort.
I would strongly advise you fcousin to draft a letter, stating that if the dress isn't returned within 28 days, a summons will be issued.
She should take this letter to the woman's house, and ask first for the dress to be returned. If it isn't, give the woman the letter, and next day post a copy by recorded delivery as well.
Hopefully that will be an end to it.