You don't mention how long ago you postdocced with the author, and this is relevant.
If the postdoc was recent, I'd be inclined to turn down the review, as there may still be papers you'll be authoring together, collaborations to wrap up, recommendations the author may write for you, ....
If the postdoc was not recent, and there are no active collaborations and/or papers in the hopper, and you're not particularly close, active friends, I'd be inclined to accept the review.
Thus, the devil is in the nuances -- how many available qualified reviewers there are in your field, how "long ago" is long ago, whether the paper is likely to become central to some big battle in your field, ....
As to "how long ago", there is no right answer, but 5 years since the last collaboration seems to be a guideline I've seen before (possibly in NIH study section guidelines as to whether you need to declare a conflict or not, but I could be wrong), but I can certainly imagine situations where that's not long enough and other situations where that's not short enough.
As to how close your friendship is, only you can judge whether a friendship can impact the review or if the review can impact the friendship. My own inclination would be to not review for close personal friends. Of course, in small research communities, where all the authors travel the same conference circles, everyone knows everyone else, and if you're strict about this, nothing would ever get reviewed.