I’ve been pondering this all day. The conclusion I’ve reached is that the explanation his friend gave was the best one.
When Tom Cruise showed up in a taxi, they knew he didn’t belong there. But he knew the password, and they had no way of knowing what else he knew, so they decided both to show him what they were doing, and also to scare him into silence, as they presumably wanted the very existence of these events secret, and he did not know, then, how badly he had erred. Between the gate and the time he pulled up, they looked at his picture, and realized who he was.
They decided to let Mandy warn him, because it was just possible he would recognize her by her breasts, which differ in details woman to woman. He would also have reason to believe that she was sincerely trying to help him, as she would have a motive, since he had helped her. They did their charade, and at the end of it she was so emotionally exhausted, she completed the suicide she had been half heartedly trying to commit for some time.
And his friend was quite sincere in saying they wanted to scare the shit out of him: hence the mask.
That movie is perhaps the best movie I have seen delving into the erotic instinct. It is on display in many contexts, in many ways. It drives so many people mad. It is also holy. Above all else, it is powerful, both in expression and repression.