First make sure her elbows are at right angles to the body. Delt benchers tend to keep their elbows in to the body. Have her maintain this position through out the entire press. You did not specify what her elbow position was. Take these two measures then write back.
Many lifters use negative reps to overload their lifts.
What they do is start to lower slowly but as the weight nears the bottom, it just crashes down. Unfortunately most lifters need the most help there. Thank You Gail. Look back at previous answers I have given. I am big on alternating rep schemes very 4 weeks or so.