Inside the venue...
It was very easy to tell that Xiu's attention wasn't on the wedding ceremony. She was solely focused on her cellphone. It's not like she was interested in wedding vows and stuff. She didn't even need to pay attention just for the sake of reference.
From Darren's position, he had the perfect view of Xiu's phone screen and that's why he could actually read the conversation going on between Xiu and Han Bohai through texting.
The very first message she sent was, 'Don't tell me you kidnapped my Ying Jie.' And soon followed another message followed with, 'And if you really did. Use this opportunity wisely.'