Well, loosely speaking Anna's power works through the medium of spiritual energy, which due to the strength of the spirit within her is fairly significant, but not absolute. Given enough time she can essentially influence water in one hundred feet in every direction. When your character manipulates water he's moving the water with his power, where-as when she moves water her 'spirit' is literally inside of it, and she's moving the spirit.
So, with that in mind, if Anna is directly manipulating water it would be incredibly difficult for your character to exact his own power on it, because every molecule is already infused with her spirit and he'd have to contest her for every inch, where-as water she isn't directly moving or controlling still has her spirit within it's not actually active. She can feel when it's being manipulated though, as was what just happened.
Point being, I'd say you're absolutely right, your character would have extreme difficulty counter-acting her movement of water when it's being actively manipulated by her, but he's relatively free to manipulate anything else. Essentially we'll just give the advantage to the attacker in every circumstance, my character (even if she has prior spirit infused the water he's manipulating) will have trouble re-asserting control over water your character is actively using, and vice versa.