Yeah, we've discussed this particular point somewhere else here. He knew who she was. He ordered her a glass of milk, and made her wait for the creme, which is considered not kosher by Jewish people. He knew that if the Basterds didn't succeed on this particular night, that she would. And if she didn't, the Basterds would eventually succeed, and he wanted a get out of jail free card to play, so he allowed them to go.