Let's say that one author has a paper under review, and continues on subsequent works based on the paper under review.
When the author puts preprints of subsequent works available online, should the author let the editor of the first paper know about those subsequent works?
That's a somewhat tough question to answer (though a good problem to have!).
It depends on the journal you submitted it to's policy on the subject. The biggest issue will be if some of the work you're publishing elsewhere will be required by the reviewers for the current project. You can find yourself in a sticky situation.
For instance, let's say they want you to do an experiment or report on data that you were planning to publish elsewhere...that data hasn't yet been published so you can't include a reference to it yet, but if you include it in your currently submitted paper during the rewrite/resubmission, then you can't submit it as new work.
Also many journals won't accept the newer manuscripts for review until you've published the reviewed work on which it was based.
Each situation is unique. Best thing to do is just ask the editor.