All good answers. I agree that swimming and some Pilates can be helpful. One of my favorite exercises, given that your client is medically cleared, is a prone back extension. Lying prone, head and neck in neutral position, lace fingers behind head, elbows out. Keeping head and neck is neutral, no movement at shoulders or shoulder girdle, tighten the lower back muscles and extend off the surface until the nipple line just leaves the surface (approx. 40-5 degrees if extension past neutral.) Hold tension in the lower back muscles throughout the recovery phase. Be sure to maintain breathing rhythm on both effort and recovery phases.