Emma washed her face in the restroom and told herself to get it together.
After all, she took months to bring Charlie to meet her family and she didn't introduce him as a boyfriend but as a fiancé right away. However, they were in different cities. It's not like she saw her family every weekend and pretended to be friends-only with Charlie in front of them.
Now Emma understands Sophia's point that between an employer and a personal assistant there needs to be a connection of trust. How can she trust Tiffany after this? Knowing that Tiffany hid for a month that she is with Mike while sneaking around, Emma is wondering: what else is Tiffany hiding? Even if she is not hiding anything now, how can Emma have confidence that Tiffany will not work behind her back in the future?
It's not that Emma didn't see signs that something is going on, but she brushed them away because she believed that if something IS going on, Mike and Tiffany will tell her.