If you can’t be kind, at least be vague.

Judith Martin (b. 1938) American author, journalist, etiquette expert [a.k.a. Miss Manners]

“Miss Manners,” syndicated column (1982-08-28)
    (Source)

Widely cited as a Miss Manners quotation, this is actually the headline given in at least some outlets (e.g., The Washington Post) for this date's column (which may or may not have been the title suggested by the column itself). The phrased in an expanded form in the article:

In any case, Miss Manners does not believe in ending a summer fling by explaining that it was a summer fling, when the other person might have considered it significant. Neither does one document the decline of one's interest; it is not nearly so charming a story as the build-up of feeling was, at the beginning of the summer.
There is no way to be kind in such an assignment, but you can at least be vague.