Snoopy65,
I'd do a water change, but wouldn't be overly concerned. Any heterotrophic microbes in the tank will utilize and breakdown the sugar, and the autotrophs will finish the rest off.
Nitrobacter and Nitrosomonas won't be affected. Sugar is actually one chemical I add to the bioreactors at work (I'm assistant superintendent for a waste water treatment plant in an oil refinery) when "food" is low to maintain the heterotroph population. Bushwhacker is right about the potential for a bacterial bloom. With the amount that was added it shouldn't be too severe if it occurs at all.
The biggest concern with a bloom of heterotrophic bacteria is the possibility of dissimilation when the "food" is gone. This is when they start "eating" nitrates in the absence of a more appropriate energy source, and converting the nitrate back to nitrite and then back to ammonia. This can potentially overwhelm the nitrifiers' ability for conversion until the population of nitrifying bacteria can catch up. Many heterotrophic bacteria are motile, so if ya get a cloudy or milky appearance in the water do another water change.
I'd be surprised if ya see any direct affect on the betta or the plants. Everything that is taking place is on a much lower level right now, and if ya did a decent water change, ya probably won't see any effect at all.
WYite