Haha, yes this has happened to me before as well, and it was also asked here in the past, in the question "full pool table pocket".
In your scenario, your friend would lose his turn because of what happened. But, he could have prevented that by having some balls removed from the pocket before taking his shot. This is allowed, but it is the shooting player's responsibility to ensure that it happens before he takes the shot. He can do this by either asking a referee to remove the balls (if playing in a refereed match) or by removing them himself (without disrupting any in-play balls) before shooting.
Here are the relevant sections of the various 8 ball rules for when the pockets are full, which support what I am saying:
BCA/WPA general rules of pocket billiards, section which say:
- Balls that don't remain in a pocket are not considered pocketed, regardless the reason, and;
- It's a scratch if your cue ball touches an already pocketed ball (e.g. in the case of a full pocket of balls).
3.11 POCKETED BALLS
A ball is considered pocketed if as a result of an otherwise legal shot, it drops off the bed of the table into the pocket and remains there. (A ball that drops out of a ball return system onto the floor is not to be construed as a ball that has not remained pocketed.) A ball that rebounds from a pocket back onto the table bed is not a pocketed ball.
3.20 CUE BALL SCRATCH
It is a foul (scratch) if on a stroke, the cue ball is pocketed. If the cue ball touches an object ball that was already pocketed (for example, in a pocket full of object balls), the shot is a foul.
UPA Tour Rules for when pool table pocket is full with balls:
11.5 Ball Rebounds from Pocket
Balls must remain in a pocket to count as pocketed. If a ball goes into a pocket and bounces back on to the playing surface, it is not considered pocketed. If it is the 8-ball, it is not a win. If it is the cue ball, it is not a scratch. Clearing pockets that are full or nearly full of balls is the responsibility of the shooting player.
Billiards: Official Rules & Records Book by Dawn Meurin (1993), Page 61, which has a section specific to "Clearing Pockets" and discusses the proper procedure to follow if your pockets are full of object balls:
Clearing Pockets: On pool tables that do not have ball-return systems, the referee will remove pocketed object balls from full or nearly full pockets. It is the player's responsibility to see that this duty is performed; he has no recourse if a ball rebounds from a full pocket.