This may be me looking too much into it, but Abir does not know that the coffee was for Mishti all he knows is that the victim in this situation is him. Very honestly, I felt Abir was right, if this had come out today, the issue would be much more magnanimous. As much as I hate what Kuhu has done up to now, if this comes out now, it will be bigger as Parul was already blamed. As he said, the time in which this truth should have come out is over, if it was out sooner there is a chance that people will accept that it is an accident. But now, even Mishti will be blamed on why she hid the issue in hand, and the drunk love we saw will backfire, as people will say she did it too protect her sister.
The issue isn't about whether the coffee was for Mishti. What's obvious is that Kuhu has lied since many days now, let others take the blame for her deeds, especially Parul.
Abir claims to be very concerned about Parul maasi but doesn't feel the need to clear up her honor cuz Kuhu may be exposed and that would then impact his Nanko. That is where his approach is wrong.....he is enabling wrong behavior.
Mishti was right to point out that Kuhu will never learn unless a price is paid for such acts....she continues to blame others and gets away with it because there is no accountability. But Abir basically told Mishti, he doesn't want to bring Kuhu's actions to justice cuz it would impact Kunal and Meenu.
So Mishti was right to feel hurt over the lack of support from Abir there.