Welcome. That does occur in poultry sometimes. If it isn't your offspring, or you don't know it is, than their instincts is to kill it sometimes as an intruder. Not all have proper instincts.
It's always best to separate out the very young for a few weeks before allowing the rest of the flock to meet the new members, and to wait until they are stronger and more able to get away from aggressive adults.
Your friend should have intervened and removed that drake, or removed the hen and ducklings the minute she saw that degree of aggression.
Not all ducks are so aggressive, but when hatching out any species of poultry it's best to be aware that it does happen and to keep an eye out, and separate when necessary, and help the hen protect her young.