If I understand the situation correctly, you work with this girl that you're really digging, yet her mom won't let you talk to her outside of work. This might not help, but all I can really suggest is that you somehow show the mom how much you care about the girl. Perhaps buy the girl flower's one day, and give them to her in front of her mom to help show that you really aren't a bad guy and you truly do care. What does the girl think about you?