She even didn't talk to Preeti much and keep distance as we seen till now. Preeti is also very much less talkative and keeps distance from Avantika. Even after insisting by her father (Mr. Diwan) and Harish she never said word about the female she suspects.
So I think Avantika must have mis-understood relation of Harish and Preeti. She must have seen something between them that she suspects that they have an affair.
It may possible, that Preeti was cheated in her younger days by some one or else raped by some one and Harish saved her or helped her when she got pregnant, helped her to abort the child and Avantika only known / saw them at hospital or read the abortion reports at doctors place or somewhere else or it may possible that some enemy of them given news to her.
I am sure Avantika suspecting Harish-Preeti affair and believes only what she understood without confirming it with both of them.