None of what you have posted so far appears enough to support defamation suits by either Steven or John, but a California attorney could review all of the facts to determine this better.
There would need to be a review of the Facebook page written by John, to see what exactly was written about Steven and his business, and there would need to be a review of the email, to see what exactly was written by Steven about John, and there would need to be a review of all of the facts that led to the defamatory exchanges, and a review of the claimed reputational injuries suffered as a result of the defamatory statements.
Steven is wrong that the contents of an email cannot be the basis of a defamation suit - if an email contains defamatory statements about someone other than the person to whom the email is sent, that is enough to cause reputational injury and potentially support a suit against the emailer.
Being called a "liar" is generally not defamatory. It is just not a very nice thing to call someone. Again, however, a review of the facts would be needed to determine if this "liar statement," taken in context of what else was said, could be seen as defamation. This would be true also for calling someone "dishonest." These are words that need to be seen in context in order to determine their defamatory nature.
As for your last comment, yes, it is difficult to prove a defamatory statement led to reputational injury if the source of the reputational injury can be equally attributed to something else. Steven the business owner would also need to prove that the statements made by John on his Facebook page were made with actual malice if, as you mention, Steven can be seen as a public figure (public figure status is determined by the court).
California has a one year statute of limitations on defamation suits, the time within which a suit must be filed after first publication of the defamatory statement. If either Steven or John are interested in pursuing a defamation suit against the other, then they should consult with an attorney in their area.
Re-read Proserpina's posts for some of the proofs necessary for an action, a good indication of costs and times that can be involved in such an action, and good advice.