Here are 3 applicable rules from the 3 common rule sets: OnePocket.org, WPA, and DCC. In our rules, the issue of accidentally moving only one object ball, in contrast to more than one, is not spelled out to a "T". However the reference to "disturbed balls" seems to imply that it is not a foul simply to move more than one object ball. The distinction seems to be whether or not the balls can be restored, or at least restored satisfactorily. If not, it is a foul.
The WPA seems to be saying that unintentionally moving only one object ball is not a foul, unless it changes the outcome of the shot; but that moving two or more is.
And the DCC is very specific that moving more than one OB is a foul. Moving only one is not a foul unless it changes the outcome of the shot in progress.
My personal preference is our own rules: the unofficial expert body of one-pocket. For example, if I'm playing a guy in a private match, and he accidentally bumps two OBs, I would just put them back where they were, and allow him to continue on.
But in the case of the original posted scenario, the balls would not have been likely able to be restored, so it would at minimum result in a loss of game to the offender. ~Doc
onepocket.org RULE: 6.1 Unless otherwise announced by the tournament director, One Pocket is played according to the World General Rules 1.16.1 [defunct], ‘cue ball fouls only’. In the event that a player accidentally moves a ball, the opponent may elect to have the disturbed ball remain in its new position or be restored to its original position. When balls are restored, they shall be placed as close as possible to their original positions, with no advantage to be gained by the offending player. If no official is available to restore disturbed balls, then the players must come to agreement on satisfactory replacement of the disturbed balls prior to continuing play.
WPA RULE: 20. Cue ball fouls only
If there is no referee presiding over a match, it may be played using cue ball fouls only. That is, touching or moving any ball other than the cue ball would not be a foul unless it changes the outcome of the shot by either touching another ball or having any ball, including the cue ball, going through the area originally occupied by the moved ball. If this does not happen, then the opposing player must be given the option of either leaving the ball where it lies or replacing the ball as near as possible to its original position to the agreement of both players. If a player shoots without giving his opponent the option to replace, it will be a foul resulting in cue ball in hand for the opponent.
DCC RULE: Cue Ball Fouls Only: Accidentally touching or disturbing a single object ball is not a foul unless the disturbed ball has an effect on the outcome of the shot. See a referee for advice if you think this has happened. Otherwise, the opponent has the option to restore the disturbed ball or leave it in position. If the offending player attempts to restore the disturbed ball without giving their opponent the restoration option, it is a foul.