I see no reason why the bodyguard should be arrested in this situation. If there is no any law that prohibits bodyguard from placing the bet, just like every other person he has the legal right to place bet on any events of his choice as the country permit. Despite being an insider, he is not a member of the election committee to know the exact date of the election. What I see here is the use of analysis based on the information he heard since he's always close to the prime minister and make use of the information to guess the correct date. Every gambler will do everything possible to win their bet.
As far as people who are very close to the sources that have access to such information are not banned from placing bet on such events, issues like this should be highly expected. Assuming an outsider make such bet and won, what would they have accused him for? Just curious to know