She probably doesn't even know Shagun was unfaithful. She just thought that Shagun left Raman and went with Ashok. She knows Shagun is manipulative and self absorbed, but Shagun has also been A LOT nicer to Ishita than Raman. All she's seen of Raman is his bad side. She's seen him making fun of her parents, she's seen him neglect Ruhi to the point where the entire family had to step in, she's seen him harass his ex-wife while drunk, she's experienced first hand his rudeness of abandoning her at a party and not even giving her a single cupboard to put her clothes in. Raman has been nothing but rude.
And fyi she didn't know that Raman was grappling with the fact that he thought Ruhi wasn't it. He's raised her for years and he obviously considered himself Ruhi's father mentally, but physically Raman did not show that. And as another poster pointed out, she probably thinks Ashok and Shagun got together after Shagun divorced. There's no question then that Ruhi cannot be Ashok's because in Ishita's mind Ashok wasn't there. On top of that, Ruhi is a carbon copy of Raman. It's obvious to everyone except Raman that Ruhi was his. In her eyes, she couldn't even fathom that Raman wasn't sure if Ruhi was his because everything matched, regardless of DNA.
And even when Raman did find out that he was Ruhi's father, he made a stupid comment about winning. The last comment Raman had made about winning was against her and against Ashok. In her eyes, Raman does not value relationships for themselves, rather places their importance in terms of winning and loosing. Which again places him in a bad light for Ishita.
I'm not saying Raman is bad. He's a very sympathetic character. But only WE know that. We've seen his sadness, his tragic backstory, his pain, his good side. All Ishita has seen is his bad side and a little bit of sadness.
Ishita is a lot harder to sympathize with because she's had a relatively easier life. Thus Raman always comes across as better than Ishita.
I agree that Ishita sometimes does not know her limits. But on the whole I think Ishita acted alright in this track.