I came across this, below, in a reputable national newspaper:
Mary Beard, the renowned classicist and an alumni of the college
Now, you don't have to be a professor of classics like Mary Beard to spot that she cannot be an alumni of a college, because the word alumni only applies to students in the plural, and even then they have to be either all male, or in a group including both male and female people.
The word for a female student of a particular college is alumna; if there is more than one female student then they're alumnae. If the student is male then he's an alumnus...
...except that I'm suddenly full of awful doubts. If we must use they to describe someone who, well, wants to be called they, then perhaps we should apply Latin plurals to singular people (and Mary Beard is a singular person) if they so ask.
I'm pretty sure that Mary Beard hasn't so asked, though, being a professor of classics and everything. So I'm happy to mark alumni down in that quotation as just plain wrong.
Nuts and Bolts: alumnus/alumna/alumni/alumnae. This Latin word means nurseling, foster child or pupil, and comes from the word alere, which means to nourish.