Poster A gave an upvoted and accepted answer to a question. Poster B also answered the question, but then deleted his answer—perhaps after a quick glance at A's short and simple answer, considering that his own was no improvement. However, A's answer was wrong while B's (deleted) answer was right. I would like to point this out to B. How can I do this?
If you know that user B routinely hangs out in chatroom C, you can drop in there and tell them about it.
If user B doesn't chat much (or not at all): abusing the comment system for such purposes tends to be tolerated (it would be nice if you clean up yourselves after the matter has been settled). Within reason. If B declines or doesn't react, don't pester them.
Generally, more important than notifying B is to notify A and the question author. (Here done, resp. not applicable.) If A fixes their answer, all is well in that regard. Especially when the account of the question author no longer exists, so the accept mark cannot be given to a different answer, that is highly desirable.