This is not unlikely at all, it's almost guaranteed in my opinion, how else will the story progress down the road if she never finds out, I guess he breaks up with her, and then him and Nicole run away together without anyone ever finding out? I doubt that she doesn't find out at some point, she will eventually find out, and it will be interesting to see how she reacts and what comes of it.
I suppose perhaps there could be an early ending of sorts where you run off together, which means breaking up with Chloe. When her mom also ends up running away at the same time though, she will likely have her suspicions anyways. Anyways this is all just highly speculative, but at some point in this game, barring an early ending type of deal, Chloe WILL find out.