Isn't the MC's dream is to become a CEO/upper management? Can she still realistically do that when she's dating someone who's the son of the company owner? Even if she claimed she didn't know until just recently are other people going to believe that? The ML was selfish in believing that once she learnt of who he was things wouldn't change, and that people would still look at MC favourably. He's literally become the biggest roadblock to her achieving her dream (using her own hard work) because even if it was the result of her efforts 100% she can't guarantee that ML hasn't been pulling strings behind the curtain to help.
On top of all that this guy has proven that he doesn't even really need this job to begin with. If he's going to inherit the company this is basically just "for fun" for him, regardless of how hard he works. He didn't even get the job because he applied and went through the process everyone else did, he was probably handed it by his parents.
ML fucked it up by thinking this was going to be some cute reveal where she's happy that he's rich and owns the company she works for. He is literally the worst person for her to be dating in this situation.