Yes, the student can take a semester (or more) off with the understanding that graduate program must be completed in a 5-year time period. If the student is not active for two semesters, the Graduate School may discontinue the student from the program and they would need to request reinstatement before being allowed to continues classes again.
The courses can be taken in any order. However, the sequence of courses are offered in a rotational schedule. If you miss a course in a semester you may have to wait until the following same semester to take the course again.
For example: Student takes Spring semester off. Two of the program courses are offered in that semester and the student needs one of them to complete the certificate. The course will not be offered again until the following Spring semester.
Schedule to speak to an advisor for further information.