Yerbol tried to stop a few times while following Lysa in order to speak to her further, but she didn't respond to his attempts, choosing instead to plow forward, shouting back at him:
"We can talk later! Sith! In tomb! Let's go!" 

When she got an idea in her head, there was absolutely no way of stopping her. So he would follow Lysa, see if this was indeed part of the trial of vengeance that the temple was constructing or maybe, just maybe...
"Here, see!" She stopped, pointing at the entrance to a rather familiar tomb. So this WAS a trial after all. How could he believed anything else, even for a moment? To think that the sight of Lysa's cold, lifeless corpse was just a mirage was a moronic lapse in judgment that he wouldn't allow to happen. Maybe if he verbalized this, he could move on from the trial and complete it? No, that wouldn't make sense. There had to be an instance of where vengeance would be enacted. This path that he was taking could only lead him to the same tomb he had fought Aria in. Would it be the real Aria or a mimic of her? After all, she had gone down the path of power. What if she had been given more power to strike him down with and was waiting for him, to get her chance to end his life like she should have done on Korriban. 
"C'mon, don't just stand there! Once we take her down, then we can get inside and I can show you what I found." Lysa stepped forward, but Yerbol shook his head. 
"No, Lysa. I'm not going to follow you." She turned on her heel, her eyes blazing. 
"And whyyyy would that be?"
"Because you're not real. I won't give into this path of vengeance you're trying to lead me on."
"Jedi." She snorted, her lightsaber crackling to life before dashing forward, attempting to slice Yerbol at the torso. He barely dodged in time, the heat of her viridian colored lightsaber perceived by his skin. He wouldn't take the chance that this version of Lysa couldn't actually end his life, the Knight igniting his saber and delivered a succession of slashes, all of which were dodged gracefully. Grimacing, he tried an overhead slash only to be blocked, Lysa's foot planting itself in his chest, driving him back. 
"You're weak, brother. You were always the weaker one of us."
"That so?" Their sabers clashed as blows were made and blocked swiftly. A pause in their skirmish allowed Lysa to answer:
"Of course you were. You never wanted to go outside of that little comfort zone Master Braga gave you. But me? I wanted more."
"That was your problem; you never knew when to stop." Lysa let out a cry as she thrust her saber forward, Yerbol knocking the saber aside and plunging his own into the false Lysa. A moment later, she was vapor. Darkness enveloped the dunes, the only light coming from his saber, which was held with both hands, his legs bent and ready to spring at danger. Instead of being assaulted by a malicious force, he was back in the hallway he had started in, the wall to his right receding. He stepped through to find himself in another torch-lit hallway, except this one had a visible opening to where they had started. He emerged, spotting Aria. A sigh of relief.
"Well, how'd it go?"