The Ares 3 crew is turning around at an specific angle and using the momentum that they already have with their acceleration to quickly head to Mars, as Venkat described. "...they’d start accelerating right away, to preserve their velocity and gain even more. They wouldn’t intercept Earth a tall, but would come close enough to use a gravity assist to adjust course." While this happens, Mark starts to make adjustments to the rover.
Around Sol 529 (estimation)
The Ares 3 crew is collecting the Iris probe that Mark was supposed to get, and going to use the food for there 2nd journey to and from Mars, which is another step that Venkat describes. "Around that time, they’d pick up are supply probe with provisions for the extended trip."
Mark has to have finished making all of the adjustments to the rover by this time, including adding the Oxygenerator, Water Reclaimer, etc. To be rescued, though, "Watney would have to get to the Ares 4 MAV," which will be very dangerous.
Mark just arrived by this time. He is starting on working on adjusting the MAV "...to lose weight…alot of weight." He also has communication to Earth during this time, which helps.
The Hermes craft, which is "[is meant to be] a flyby" is flying over Mars right now. Mark will be launched into space and picked up by the Ares 3 crew. Then, the Hermes ship turns around and head to Earth again, retaining all of their momentum.