Meeting at the Airport
After convincing her family, Scarlet, then, contacted Wil and informed him that she agrees to his conditions, however, requesting two things at the same time.
One is to keep Rey's birth secret from Scott and Mia, and the other is to change his birth year from 2001 to 2000 as she already committed to his family that way, so that, there won't be any confusion regarding the difference between their age, especially to Scott.
In fact, she wanted to lie with the same thing to Rey and keep him in dark. However, when she remembered Wil's wealth, she was worried that in the future, Rey might claim Philip's inheritance too and fight with Wil.
At that time, when the truth will be out, he will only become a fool in the end. It is something that she wasn't willing to see, ever.