She didn’t. Before he asks, She says it’s okay he isn’t like Zack because she wants to be with Cloud. When Cloud does ask, Aerith says “I think so? But not about anything important” which means whatever the conversation she had with Tifa about Zack was just that… not important.
Cloud's mental instability is absolutely important. Her side-stepping his question by pretending it's not important is where she is lying. She knows it's important, but is lying to protect him, same as Tifa.
This whole "lying" thing is a bad angle and I don't know where you plan on going with this. People lie to each other all the time for a variety of reasons. I've lied to my parents, my siblings, and my girlfriend before. And they've all lied to me. But we still love each other. These characters have good reasons for lying/hiding the truth, or at least believe they do, same as any person in real life.
It feels like you just brought it up to trout out a "yeah, Tifa might be the winning girl but she's a terrible person and they're a terrible couple because look at her lying to him" argument.
Also, this is a general comment directed towards everyone posting in this topic currently, but I'd recommend against posting "hearsay" spoilers that haven't been confirmed by reliable sources. "I heard this was true" just makes your argument look weak. And I've seen some compilation stuff dug up too, but there is a separate topic dedicated to that. This LTD topic is for the Remake trilogy only.