My daughter is getting married and is making her own centerpieces with branches & baby's breath in a vase with water and floating candle. She has made a couple to see how they will look and find that after a few hours, the branches and baby's breath start to break apart and the bark comes off floating in the water. This makes the centerpieces look funny. What can she do to stop this from happening? Should we spray the branches and baby's breath with a waterproof coating of some kind? Or is there something else that we can use to preserve them for the entire day?

DRIED branches and dried baby's breath? They will just start to decompose in water. If it looks nice when first done, then plan on them being made at the last hour.
You can buy spray waterproofing at a craft supply place, or maybe a florist.