The smoking gun was definitely her admitting she hoped she was helping Charles. This would not fly in the US.
Collusion doesn't fly in the U.S. (and is explicitly forbidden). Forfeiting your chances of winning in order to help or hinder another player -- so long as it wasn't done with the expectation of sharing in the champion's winnings -- is permissible so long as the rules of the game are followed.
See, for example, any
Press Your Luck contestant who was in third place and passed the final spin. Or that
Wheel of Fortune contestant from 2015 who started making terrible guesses in the speed-up round (for reasons that were never revealed, although one hypothesis was that she wanted one of her opponents to win additional consolation money). Or a
Price Is Right contestant deliberately overbidding on a showcase because they already won enough nice prizes and they want their opponent to win something.
If it's possible for a game show contestant to ruin another contestant's chances of winning through legal in-game actions, then it indicates a flaw in the game and/or a flaw in the contestant selection process. It's true that what Lainie did goes against the spirit of
Temptation, but that's the risk the producers took when they played a game in which an incorrect answer immediately ends a scoring opportunity. The spirit of
Jeopardy! is to play to win, but when too many contestants started wagering to tie rather than add that extra dollar, the exploit was patched. But until that time, playing in a manner not in the spirit of the game is completely legal.