"Actually, I didn't know that from the beginning."
Suizi, of course, couldn't have possibly known beforehand. She derived it step by step based on the situation.
Yu Jingting was often drunk during the day, likely gathering those underlings of Ma Guocai before his imprisonment. He could only meet with Ma Guocai's wife with a crowd, and give her money in private.
This assumption stems from Suizi's understanding and trust in Jingting.
With Ma Guocai behind bars, if Yu Jingting were to meet his wife alone, it could breed suspicion. Plus, Jingting wouldn't fully trust Ma Guocai's wife either.
In such circumstances, the best approach is to find a bunch of witnesses. This is also an unwritten rule in the circle.
Jingting, having grown up as a street kid, is adept at navigating complicated social rules, even after moving from a rural area to the city.