Hal has alluded to the "Rules" covering the issue:
11. BALL IN PLAY
Unless a fault or a let is called, the ball is in play from the moment the server hits the ball, and remains in play until the point is decided.
USTA Comment 11.1: Is a point decided when a good shot has clearly passed a player, or when an apparently bad shot passes over the baseline or sideline? No. A ball is in play until it bounces twice or lands outside the court, hits a permanent fixture, or hits a player. A ball that becomes imbedded in the net is out of play.
24. PLAYER LOSES POINT
The point is lost if:
a. The player serves two consecutive faults; or
b. The player does not return the ball in play before it bounces twice consecutively; or
c. The player returns the ball in play so that it hits the ground, or before it bounces, an object, outside the correct court; or
d. The player returns the ball in play so that, before it bounces, it hits a permanent fixture; or
e. The receiver returns the service before it bounces; or
f. The player deliberately carries or catches the ball in play on the racket or deliberately touches it with the racket more than once; or
g. The player or the racket, whether in the player’s hand or not, or anything which the player is wearing or carrying touches the net, net posts/singles sticks, cord or metal cable, strap or band, or the opponent’s court at any time while the ball is in play; or
h. The player hits the ball before it has passed the net; or
i. The ball in play touches the player or anything that the player is wearing or carrying, except the racket; or
j. The ball in play touches the racket when the player is not holding it; or
k. The player deliberately and materially changes the shape of the racket
when the ball is in play; or
l. In doubles, both players touch the ball when returning it.
As far as the contention regarding the "etiquette" implying or dictating what a player should do or is expected to do, there is "The Code" which actually addresses what proper etiquette actually is:
PART 2—THE CODE
THE PLAYERS’ GUIDE FOR UNOFFICIATED MATCHES
PREFACE
When your serve hits your partner stationed at the net, is it a let, fault, or loss
of point? Likewise, what is the ruling when your serve, before touching the
ground, hits an opponent who is standing back of the baseline. The answers
to these questions are obvious to anyone who knows the fundamentals of tennis,
but it is surprising the number of players who don’t know these fundamentals.
All players have a responsibility to be familiar with the basic rules and
customs of tennis. Further, it can be distressing when a player makes a decision
in accordance with a rule and the opponent protests with the remark:
“Well, I never heard of that rule before!” Ignorance of the rules constitutes a
delinquency on the part of a player and often spoils an otherwise good match.
What is written here constitutes the essentials of The Code, a summary of
procedures and unwritten rules that custom and tradition dictate all players
should follow.
No system of rules will cover every specific problem or situation
that may arise. If players of good will follow the principles of The Code,
they should always be able to reach an agreement, while at the same time
making tennis more fun and a better game for all. The principles set forth in
The Code shall apply in cases not specifically covered by the ITF Rules of
Tennis and USTA Regulations.
Before reading this you might well ask yourself: Since we have a book that
contains all the rules of tennis, why do we need a code? Isn’t it sufficient to
know and understand all the rules? There are a number of things not specifically
set forth in the rules that are covered by custom and tradition only. For
example, if you have a doubt on a line call, your opponent gets the benefit of
the doubt. Can you find that in the rules?
Further, custom dictates the standard
procedures that players will use in reaching decisions. These are the
reasons we need a code.
—Col. Nick Powel
Note: The Code is not part of the official ITF Rules of Tennis. It was meant to
be used as a guide for unofficiated matches. This edition of The Code is an
adaptation of the original, which was written by Colonel Nicolas E. Powel.
MAKING CALLS
5. Player makes calls on own side of the net. A player calls all shots landing
on, or aimed at, the player’s side of the net.
6. Opponent gets benefit of doubt. When a match is played without officials,
the players are responsible for making decisions, particularly for line
calls. There is a subtle difference between player decisions and those of an
on-court official. An official impartially resolves a problem involving a call,
whereas
a player is guided by the unwritten law that any doubt must be
resolved in favor of the opponent. A player in attempting to be scrupulously
honest on line calls frequently will find himself keeping a ball in play that
might have been out or that the player discovers too late was out. Even so,
the game is much better played this way.
10. Treat all points the same regardless of their importance. All points in
a match should be treated the same. There is no justification for considering
a match point differently than the first point.
http://dps.usta.com/usta_master/usta/doc/content/doc_13_2292.pdf
To the OP should that happen not only does the player struck by the "live ball" lose the point by Rule, he/she is obligated to make the call against him/herself by Code, which describes much of what is considered proper etiquette on court.