I suggest you speak with the airline the bride will be flying on. Your friend is not the first to be wed in the D.R and the airlines MUST deal with this all the time.
I once flew over on a charter flight from the UK and noticed that a couple had packed the wedding dress in an ordinary cardboard box and put it in the luggage hold.
I carried my wedding dress on board from VA to LA. I do believe as the airlines like to know if you are going to carry it on so they can accomodate your dress. We had no problems.
As a retired Bridal Consultant and still Bridal Shop owner.......
"Wud" has it right!
Ask the shop where the dress was purchased to pack it for travel (they should). How we pack them depends on the style or the dress - train length etc. but either bagged or boxed the gown is carried on and stored in the in-cabin closet available for such items.
DO NOT (I repeat -DO NOT) have it put in the cargo hold as it may not arrive.............!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Congratulations to your friend and Best Wishes to her!