Isn't it actually the bowlers end umpires job to spot a no ball. Sq leg ump can assist by guiding but it's not his call.
Yep, spot on
(b) Bowling of high full pitched balls
(i) Any delivery, other than a slow paced one, which passes or would have passed on the full above waist height of the striker standing upright at the popping crease is to be deemed dangerous and unfair, whether or not it is likely to inflict physical injury on the striker.
(ii) A slow delivery which passes or would have passed on the full above shoulder height of the striker standing upright at the popping crease is to be deemed dangerous and unfair, whether or not it is likely to inflict physical injury on the striker.
7. Dangerous and unfair bowling - action by the umpire
(a) As soon as the bowler’s end umpire decides under 6(a) above that the bowling of fast short pitched balls has become dangerous and unfair, or, except as in 8 below, there is an instance of dangerous and unfair bowling as defined in 6(b) above, he shall call and signal No ball. When the ball is dead, he shall caution the bowler, inform the other umpire, the captain of the fielding side and the batsmen of what has occurred. This caution shall apply throughout the innings.