It wouldn't be the first time Nyx did something like that though, there's no specific point where you agree to be Layla's boyfriend for example, it just happens if you have enough relationship points, it would be a similar thing with Mel.
My guess is first part of the scene is true, MC did visit, they did have some drinks.
After that, Melissa got completely wasted and blacked out.
The rest is her drunken dream.
MC was worried for her, brought her to her bed, then stayed in chair or such to watch over her.
So he might be close to her in next update, but no cheating.
As for why she's missing her bra, she took it off herself while in bed. Or perhaps even before going to bed. Drunk people do drunk people stuff.
So far, don't think there's a single actual cheating where you don't get option to not go through with it.
This would be first such instance, don't think Nyx would do that...