Heptapod communication, and later on Louise, is performative. Imagine that you're playing a part in a live theater. You've read the script. You know what's going to happen next. You have lines to say, and you say them. You choose to participate in the performance.
It's a really interesting resolution to the argument between free will and determinism.
In this case, we can infer that, even though Louise knew everything that would happen, including her daughter's disease, she chose to play her role anyway. She understood that it was just a smaller part of a much larger, pre-determined script, and she was content to be an actor in it at all.
For another example of this being hinted at in the movie, Abbott knew exactly the fate that awaited him. He could have avoided it, but didn't. That was his role.
It's a metaphysical argument that's deeply unsatisfying to a lot of people. For a lot of folks, free will means being able to force change as you see fit -- the Edge of Tomorrow position. But, for a minority of others, free will and determinism can coexist if you're willing to accept that just because things can be changed doesn't mean they should be. It's the ultimate form of stoicism.
(Also not addressed in the movie is the neat question of whether the heptapods' original decision to interact with humans for the sake of their own future is part of the script, or if it was an action they took outside of the script for their own sake.)