Yes, whey protein can be helpful for elderly people also. The Journal of The American College of Nutrition, JACN, reports that the elderly have greater protein requirements than younger adults. Given that your mother is exercising, ingesting whey protein (or other biologic sources of protein), especially immediately following exercise, may well help her minimize age related muscle wasting. Here are some references:
Protein tissue accounts for 30% of whole-body protein turnover but that rate declines to 20% or less by age 70. The result of this phenomenon is that older adults require more protein/kilogram body weight than do younger adults.
High levels of dietary protein will not affect renal function in an individual who does not have predisposing renal disease.
.. the requirements for exogenous protein is at least 1.0 gram/kilogram body weight with additional amounts needed to heal wounds or pressure ulcers, to fight infection, or to build new tissue to replace normal losses
Regular exercise will stimulate protein tissue turnover and maintain muscle mass. However, it is essential that adequate energy and protein be provided from exogenous sources so that muscle tissue and other protein components can be restored.
According to a study in the January 2008 issue of "Current Opinion in Clinical Nutrition and Metabolic Care",
Elderly individuals are still able to respond to both resistance training and the anabolic signals provided by protein ingestion, provided specific amino acids, such as leucine, are present. Whey proteins are a rich source of these essential amino acids and rapidly elevate plasma amino acids, thus providing the foundations for preservation of muscle mass. Several studies involving supplementation with whey protein have been shown to be effective in augmenting the effects of resistance exercise, particularly when supplementation occurs in the hours surrounding the exercise training.
Even though your mother's exercises may not be very intense, it sounds like she is on the right track and the addition of whey protein following her exercises can help maintain muscle mass and should not hurt (unless she has kidney disease as @Evain Plaice pointed out).