Take a look at this post here directly on point.
Here’s an excellent response by @dpfromva [edit appropriately]:
Also @jacquo:
I understand the potential benefit of an additional data point by contacting them, but the point is that they didn’t contact you when the accident occurred or on a timely basis. Contacting them would make it awkward for you if the guest claims he didn’t know, or just very recently found out, and profusely apologizes. Then what? The point for future Hosts to know is that you were not alerted to the accident so that you could have taken timely action. So, no, I wouldn’t contact guest.
In your case, since you have the waterproof covers and an extra comforter/duvet cover, there’s no damage issue. But your review will be a welcome heads up for the next Host to prepare.