Hi Barbara
It sounds like this user may have added the course to a learning plan at some stage
if this is the case then the user is not enrolled in the course until the learning plan enrolment method is added for the course
There is a more detailed description of the actions and settings around learning plan enrolments
i am not sure of the sequence of events but check that the enrolment plan enrolment method is enabled and the user launches the course from their learning plan to access the course.
regards