Yes, she can kill him, you need to get another cage and separate them immediately before that happens. It is terrifying to see that, I have experienced that, seeing 2 fight in mid air tumbling to the ground. He is most likely intimidated by her and that is why you see him as timid and scared of everything, it is not good for him to be under the stress from her bullying. There is never any guarantee that when you place 2 birds together that they will get along and you have to take steps such as housing them separately if they don't, the cages can be next to one another but each needs their own space. It could be that she has a dominant personality or it could also be a hormonal issue either way they need to be separated before a very serious injury or death occurs.