Standing and sitting are basic tasks that become increasingly difficult with age or frailty. Assisting these movements using humanoid robots is a complex challenge, particularly in determining where and how much force the robot should apply to effectively support the human's dynamic motions. In this letter, we propose a method to compute assistive forces directly from the human's dynamic balance, using criteria typically employed in humanoid robots. Specifically, we map humanoid dynamic balance metrics onto human motion to calculate the forces required to stabilize the human's current posture. These forces are then applied at the appropriate locations on the human body by the humanoid. Our approach combines the variable height 3D divergent component of motion with gravito-inertial wrench cones to define a 3D balance region. Using centroidal feedback, we compute the required assistance force to maintain balance and distribute the resulting wrenches across the human's body using a humanoid robot dynamically balanced according to the same criteria. We demonstrate the effectiveness of this framework through both simulations and experiments, where a humanoid assists a person in sit-to-stand and stand-to-sit motions, with the person wearing an age-simulation suit to emulate frailty.