Maybe it has something to do with Reiju bleeding in the first place. I can't imagine it was part of the plan to tell her the plan. So Reiju might've been investigating and stumbled upon Pudding making preparations for the wedding.Great chapter. I wonder why Reiju was acting so oddly calm and nonchalant when Pudding revealed Big Moms intentions with her and the Vinsmokes.
If Reiju already had a hunch, she might have some counter measures in place. Or she just realizes she is up against a yonkou and is screwed anyway.