After the prep party:
And if it's instead sometime before the prep party I looked in that direction as well:
Mainly did this because with you saying "after the party" I was not sure if you meant the prep party or the last DIK party, so this is basically the entire timeline from the DIK-Party to 2 mornings after the prep party and I saw no scene with Riona and Heather talking.
In fact you find out when Heather found out that the HOTs are not gonna join the DIKs because the person that tells her...is you. In the prep bathroom she calls you stupid for even looking for the HOTs because...
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I'm not gonna rule out that the scene you talk about exists somewhere because it might be something you only see making a weird choice in a specific path or something, but Heather and Riona don't really interact on rooster during this time (except for Heather saying "Love you, sis!" when Riona asks for a toast) either, so I'm not really sure when it would happen.