It takes about 10 minutes to drive from Logan to South Station. Including time it takes the Uber to get to Logan, I'd say she's fine if she's ordering it by 7:30 AM. She should tell the Uber driver to drop her off at the bus station area at South Station. If it's between 7:30 and 7:45, she still should have time to take a taxi, since there's usually no wait for those.