Years ago on the TV show Mythbusters, they tested multiple anti sea sickness remedies. As VentureMan mentioned, ginger capsules were the winner. Ginger capsules are easily found in any pharmacy (e.g., Walgreens) in the vitamin section. I've even taken them when feeling nauseous at home and they worked pretty fast.
Two other helpful items are:
- Eating a green apple (can usually be found in the buffet). This was mentioned by the Carnival employee conducting the ship's tour.
- Looking at the horizon (something to do with equilibrium). This may help decide whether to go up on deck or to get a balcony/ocean view stateroom.
As for stateroom location, as others have mentioned, low and mid ship has the least movement.
Wishing you and your DIL all the best on your cruise!