A Cesarean section (C-section) procedure that has healed properly should not affect a woman’s ability to perform core exercises. Typically, after the 6-week post-delivery doctor’s visit, the new mom is given any contraindications and/or limitations to getting back into regular physical activity from her ob/gyn. The guidelines usually include easing back into abdominal exercises with a very low volume initially and then progressing slowly from there.
Since your client had a C-section over a year ago, you should be able to progress her through a core exercise program the same way you would any other client who is new to exercise (i.e., progressing frequency, intensity, and duration of exercise in an appropriate, progressive manner). Any core exercise commensurate with her ability is acceptable. As such, the client should be coached on proper form and technique, and if she demonstrates an inability to maintain the form of a core exercise, the movement should be regressed to a point that she can successfully complete. Of course, if pain or unusual abdominal discomfort is provoked by the introduction of a core exercise, the client should be referred to her physician for evaluation before she continues to rule out any complications in healing from the C-section.
That said, in my experience, many clients who have undergone C-sections often report that they don’t feel a connection with their lower abdomen like they do with the rest of their core muscles, even years after having the procedure. This most likely has to do with the surgeon cutting through skin, nerves, ligaments, and fascia and separating the muscles in the lower abdominal area during the C-section procedure. This results in scar tissue and decreased proprioception in that area. In these situations, it usually takes longer (weeks or even months) for the client to develop an awareness of contracting the deep abdominal muscles, and thus stabilizing through the core. Time, patience, and continued practice are the keys to helping clients who have had C-sections effectively perform exercises for their core, but it definitely can be accomplished.