Hi there Maggie,
Not sure what age an “older woman” is here so my 2 cents as a Pilates instructor and PT:
As mentioned by the other pros, the reformer involves resistance training as does mat band and ring work. So does standing Pilates: great for balance and strength.
If someone was successfully doing the above 3x a week, depending on the person it may indeed be enough but again as mentioned, we have other variables: bone loss, osteoporosis (contraindicated exercises), age, body weight, joint mobility and ability to perform the exercises?
Hope that helps.