Technically strength training is interval training; but I am assuming she is talking about cardiovascular interval training?
Regardless of age, I believe that both constant state aerobic exercise and cardiovascular interval training are beneficial for heart health. If you only do one form or the other, your body will adapt to that type of training stimulus and plateaus will result, so I recommend a mix of both forms of cardiovascular exercise. I also recommend both as anyone with an active lifestyle is bound to do both types of activities outside of structured workouts, thus they are both functional an applicable to such real-life activities.
Of course, it goes without saying that the frequency, intensity and duration of the training should be based on the abilities and fitness level of your client.