Danielle and Jerrie assumed that he was there to rescue them until he threw Grace back into the compartment with them.
All three young women got a good look at his face, and thanks to Grace sharing what happened, all three know that he killed a man.
(Way to go, Grace!)
Legarski was wearing his Montana highway patrol uniform, and in fact was almost able to convince Grace that he was trying to help her before she followed her instincts and stuck a fishhook in his face.