It's a bit ambiguous because the word is "浮気する" which can mean cheat but can also mean more generally flirt or fool around. I think it's more likely she was (mis)remembering what happened in this story
Answered you on reddit already but I'll copy it here in case anyone else is wondering about this:
The "bring your own" is a reference to events in the light novel volume 3, which was changed significantly in the anime. A similar version of events is around chapter 5 of the Ore no Kouhai manga...
Ayase if is here https://mangadex.org/title/53c77a9f-0de2-4ad6-b51a-b5022067fd5c/ore-no-imouto-ga-konna-ni-kawaii-wake-ga-nai-ayase-if but the translation stopped
Hi, great work translating Kuroneko IF into polish, but just a heads up my name is black cat knight, not black cat night. I think you have mistyped it on your cover page