Here I reference chess. USCF Rule 20E deals with this. Part 1 of the rule basically says that for solicited advice, you get a forfeit. Part II is unsolicited, and it's a lot trickier. Highlights: if the advice is bad, or if given to somebody who's likely to find it anyway (i.e. suggesting a very obvious recapture to a master), no penalty. If the advice is good, but not the only good thing, they suggest making some other move being played. If the advice is good, unlikely to be found, given by a strong player, they recommend disallowing the suggested move. If it's good, unclear as to whether it would have been found, it basically tells the TD that he has no good options. Of course, it also says that if the advisor is related to the advisee (coach, family, friend, etc.), then harsher penalties are in order.
In practice, this is a potential problem, and because of that, you keep spectators to places where they can't interfere like this.