Women need strength training, cardio, and flexibility in their exercise program. How much of each is dependent upon the individual, goals, and time available—but in general, strength training of the body should be done 2-3 times per week. I would recommend strength training as a compliment. Pilates classes can vary a lot–so if it is providing that person with an overload and proper resistance, then it will strengthen muscles and bones. Keeping a mix each week of different modes, whether that is Pilates, strength training, walking, aerobics classes, etc… helps to maintain both interest and reduce the risk of injury from any one repeated program.
Keeping the body strong with strength training (safely and with proper progression) helps keep activities of daily life more manageable. You can gain muscle at any age, so it’s never too late to start.
PS–My oldest client right now is 75 years young and loves strength training, particularly cables and TRX exercises. 🙂
Hope this helps.