I imagine her plan was to flit over, heal, and flit back. I tend to keep moving as Mercy, because it is rare that the team does much to help keep me alive. You're all shielded up with an immovable turret, so what happens when a Genji shows up? Mercy dies is what happens. Frankly, I'm not a fan of sheparding Rein with Mercy. He's usually not dealing any damage, he's generally not taking damage, he's not good at countering the flankers who are going to kill me, and when he is taking damage it's because his shield broke, and now I'm Mercy standing in front of everyone on the other team that was shooting at him. Reinhardt needs a Lucio babysitter, plain and simple. Healing, speed, and he can fire from behind your shield, as well as protect you from ults with Sound Barrier.