Well, given context, they could use any word at all, even the opposite of the correct one, and could be understood. The problem is that it seems foolish to allow that the person is correct in using the incorrect word, or that the word in question now means the same as the correct word. Effectiveness is a continuum, too - would they have communicated more effectively by using the correct word? Then that word is preferable, though not necessary to impart the meaning in the greater context.