How would you go about conveying a message to an LTR, without coming off as mate-guarding?

LTR went to movie with a guy. I want to express that it's not cool. At the same time, I want to avoid mate-guarding.

Her: "Just so you know I'm going to a movie with (insert guy)"

Me: "Have fun."

So far, that's 100% of the interaction

(I'm not concerned if she cheated or not.) Advice would be appreciated!