Usmagazine.com said the 15-year-old starlet went back to work Friday.
She was hospitalized and treated for what her doctor described as a "severe, potentially life-threatening throat infection." Her physician said Wednesday she is expected to make a full recovery.
"Oh, she's better," Momsen's co-star, Kelly Rutherford, told Usmagazine.com. "She's good. You know, she's young, and she's working very hard. ... You know, all it takes is to get a flu, and then you work a lot of hours, and you have (a serious) flu."
Co-star Jessica Szohr added: "She's fine. She's OK, and that's all I'm concerned about."