Not watering potted plants

Hello,
We flew abroad for a month and left a couple of friends to “look after the house.” They saved rent by doing so and in return were supposed to take care of the dog (expenses for walks, etc.), and water the potted plants. In addition, we gained some protection from thieves. It turns out that they forgot to water one of the potted plants and it died. We asked over the phone if they watered it and they said yes.
We had a general argument and I would like to make sure they keep their money to a minimum. Are they obligated to pay for the dead flower pot?