It seems to me that her mother must be trying pretty hard to support the two of them, and Seto can't bear asking for more. If she did, she'd see her mom even less than she does now. That's why she was two-timing those dudes. As it is, she hardly has enough money to eat, but after what she's been through, she knows her mother would be heartbroken to find out about what happened, and those happy times they used to have together are gone.