Adjective for "choosing not to speak"

"Mum" or "tight-lipped" would be my choice for that meaning. But in the context you present, "reticent" works better.

Whilst I agree that shy and bashful are primarily indicative of a person's inherent tendency not to speak out, I think reticent applies far more to specific behaviour in context.

OP is simply mistaken in thinking reticent doesn't convey the "rational choice" meaning he seeks.

Perhaps mute could carry the connotation you are looking for.

Reticence connotes a person’s character, someone who is predisposed to be quite private, somewhat like taciturn. If you are trying to convey that she was not held back by such a predisposition, then “not reticent” would be fine.

But since you are trying to convey that she was not held back for a particular reason, instead I suggest “wasn’t restrained” (or “was unrestrained”). Another possibility is “wasn't guarded”. Or a different approach would make clear that her decision was based on reason: “she had no misgivings”.