Justin thinks that Mia's the strongest woman he's met in his entire life because of how she reacted to the news. Yes, Mia yelled at them but she didn't even shed a tear and Justin doesn't mean that he wants Mia in tears, he's just a bit surprised that she handled it so well.
Not every woman that's been told her husband of over fifteen years has been cheating on her can still stand on her feet but Mia did that.
What if she already knew? What if dad's done this before and I don't know about it? Justin thinks, that's a good theory. Justin thinks that Mia might have already known that Bryan cheats on her so Justin decides that he'll have to ask Mia himself.
Justin spends all day at home since he's grounded, Mia doesn't even want him helping out at the restaurant today. She dismissed him by saying that she "has enough employees".