Here's another angle to consider: Lying does her a disservice. Maybe she is trying to improve. The coach help that happen without your candid feedback.
I've been through these types of executive coaching things before, and the coach won't share each interview with her; the coach will share aggregate findings across all interviewees. They will call out specific phrases, but in relation to a specific topic.
So the Coach will say things like, "Your team has found you to be quick to decide on a course of action, even when other ideas haven't been fully fleshed out. They say, 'she's a quick decider, but does so without all the facts'."
The first part of the sentence is an aggregation of the feedback, and the second part reinforces the coach's point with a direct (or slightly edited) quote.
Be straight with the Coach. Tell the truth. They will decide what to do with the responses they get, and how much weight to put on each individual's responses. Who knows ... they may choose not to use your feedback at all, if it is too far divorced from the other responses.
Lying always comes back to bite you eventually. In this case, you could lie and wind up with an emboldened boss who thinks she and her techniques are perfect, or tell the truth and wind up with a boss who actually improves and becomes easier to work with.