Oxford dictionary on alumnus:
a male former pupil or student of a particular school, college, or university
Oxford dictionary on dropout:
a person who has abandoned a course of study or who has rejected conventional society to pursue an alternative lifestyle
If someone chooses to quit his/her study in college, I can say that he/she is a former student of that college. Therefore, I can call her alumnus according to the definition above. It also matches the definition of a dropout.
Which one should I use: an alumnus or a dropout?