I think someone is soon going to find out what the word "dowager" means, as in "not the empress any more".
She has power because she was the empress and had authority back when he was just a little kid. So OK, her husband dies, lots of people still answer to her, not to the kid, even though he's "the emperor". But the kid has been governing for a while now, working fairly hard at it, and has turned out to be pretty competent and charismatic. The reins of power will by now have shifted more into his hands. She can either step back gracefully or be pried from power kicking and screaming, but it'll work out the same.