There's a car crash scene in the trailer.. I wonder if it'll just be her in the car and she's brought into the hospital on life support and Bruce is called in because she's going to die and is her last relative.
I would imagine that's when they reveal the healing properties of the Hulk and he makes the decision to give her a blood transfusion to heal her body. Since he's got the transformations under control he isn't worried about her turning into a monster. That being said, he probably has her in observation to make sure she doesn't go ill/turn into a monster.. and when she appears as She Hulk for the first time he feels obligated to train/guide her since he got his transformations under control.