If there is a rule that a coach cannot instruct the skater while performing, then I'd say this is a no. That's not okay.
But is this really a rule? I've seen coaches at major international coaches yell "push" or "speed" to their skaters. I've also seen them smack the boards after 8 revolutions is reached- clearly signaling the skaters.
At local competitions- you hear coaches yell "shouders" or "power" or whatever. It's not as helpful to a skater as counting revolutions, but it is in the same vein. I've never thought anything of it. The judges may think less of the skater if they hear it though.
ETA: Well, how about that. There is a rule, USFSA Rulebook 1312 (not sure about the ISU) -Clarice found it while I was editing!-
However, coaching or instruction is not permitted during the actual performance of the skater or team before the judges ... For the purpose of this rule, coaching shall be considered any communication between the skater or team and an instructor, parent or
any other person.
So yeah- I'd say that is clear violation. As is a coach yelling- "faster", "smile" or anything else. What's interesting is a parent can't do it either. I've heard lots of parents count revolutions at local competitions, but not coaches (guess the coaches know the rule!)