Actually, he is just flat wrong. You aren't dividing a finite number by infinity; you are dividing infinity by a finite number, which leaves you with infinity. In other words, if there are an infinite number of planets, and a finite percentage of them are inhabitated, there must be an infinite number of inhabited planets.
The ironic part of his whole argument is that, should it hold true, he would actually be proving with absolute certainty some form of Creationism. DeCarte proved that one thinking must exist (even if all of the physical world were an illusion, someone is being shown that illusion, and therefore, that someone exists in some sense), and given that arbitrary self-exceptionalism is rightly excluded in logic (oneself isn't the exception to the rule unless there is an adequate reason to suspect oneself is), this must be true of other thinking entities as well. Therefore, anyone you meet cannot be a product of a deranged imagination.
If, however, the probability of an inhabited planet were actually zero, as he suggests, there cannot be a natural explanation of inhabitants anywhere. Therefore, there must be a non-natural or supernatural explanation for the fact that we do exist, even if the actual nature of our existence is outside of our ability to perceive. Thefore, if he is right, an inhabited planet required Creationism.
Therefore, unless this is his intention, he may wish to abandon this line of logic. And, if he were to, Creationists would be wise to avoid this argument, as he still has his math backwards.