the person that criticised elder girl was mistaken, it is possible for anyone to forget words occasionally,[ professionals in many musical genres have occasionally done it ] I also think criticism should not be given publicly unless specifically requested.
As far as I am concerned the only thing that is unprofessional is to not bother to practise, every human can make an occasional mistake for a number of different reasons, tiredness, biorhythms etc, the most important thing is the willingness to improve,and the realisation that practise is important.
By continually using crib sheets, the person is in my opinion admitting defeat, Rather than trying to overcome a confidence or memory problem.
Finally taking deep breaths before one sings is a very useful way to relax oneself it also has the benefit of sending oxygen to the brain,being conscious of diet and avoiding certain foods and cigarettes can also help with the problem for singers of catarrh related problems