I'm unwilling by act or omission to knowingly indicate that I believe something that the Church teaches to be false. Thus, it depends on whether or not I'm being asked to participate in a deception, which I will not do. On the other hand, unlikely though it may be, if it's somehow clear that "Kevin" has no intent to deceive and is not deceiving anyone about her sex then I don't have an absolute moral objection to calling her that. This scenario is contrived and unrealistic, but it is largely a matter of prudence. Thus if I did surprisingly find myself in a similar circumstance, my actions would depend on the details.

As another Catholic commenter said, we owe Christian charity to all other human beings, including those affected by gender dysphoria. However, charity doesn't mean being "nice" or "accommodating," but it does require respecting the dignity of the human person. One way to respect that dignity is by not encouraging or condoning disordered behaviors or beliefs. I wouldn't offer a recovering alcoholic a drink, even if it was really great stuff.

That leads to another pragmatic matter. No matter what disordered beliefs or behaviors a person has (and I have my own share), we should want to help that person come to a rightly ordered place. There's really no one size fits all approach to that.