Even by Japanese standards, I don't get what she's up to here. Either she can still have some relationship with Shio, or she can cut off ties. If she's cut off ties and doesn't want her son talking to Shio, she has also cut off her familial authority over Shio. She can tell her son not to be in contact with Shio, although it's overbearing and annoying. But surely she can't tell Shio to do jack shit, although it seems like she's about to try it on.