As soon as Aria saw her boss walking in with a woman in his arms, she quickly went to open the door to his office.
She realized that this woman was different from his whores. She was dressed decently, even in a suit. Aria did not also miss the worry lines on her boss's forehead.
When Robin got closer, she saw the woman's face clearer, realizing it was his ex-wife. Aria was shocked but had no idea what had happened for him to bring her here.
She could not also conclude that her boss was fucking his ex-wife because the woman looked too decent for that.
Robin could have taken Sabrina to a hotel but figured that would send out negative misprints about his ex-wife, especially with the news about her fake relationship everywhere on the internet.
He could have also sent her to her office but since he has never been there before, he couldn't.