Because i dont think she is a spy, but instead she is the only tobi roppo who knew if Yamato was a woman. You can see Yamato's hairpin either in the last chapter, its look like similar to Black Maria.
That's what I'm trying to say.
Basically Maria might've been the last call for help for Kaido to stop Yamato from changing gender or acting like a man in 100
The moment where she rips the clothes look feminine for me. Plus her hairstyle and etc. Maybe she actually knows this all from Maria's teachings.
Btw. Bro, did you ever consider an option where Yamato is not Kaido's child but some kid that Kaido basically picked to "train" as his child?