Comma, colon or either in my text?
Solution 1:
The Oxford Manual of Style lists one of the uses of the colon as:
The colon points forward: from a premise to a conclusion, from a cause to an effect, from an introduction to a main point; from a general statement to an example. It fulfils the same function as words such as namely, that is, as, for example, for instance, because, as follows, and therefore.
None of these seem to be the case here, although I can understand the temptation to assume that it is. I suspect that the author is correct.