I've used chlorhexidine recently after gum surgery, and one unpleasant side effect is that it will darken your teeth with anything more than occasional use. Then you either have to brush like heck for weeks to get it off or go get another tooth cleaning at the dentist. Not sure if it's related to the chlorhexidine, but also noticing temperature sensitivity in the teeth around where I was swishing the chlorhexidine.
Would be better just to avoid sweet things, though. Then your friend wouldn't have to worry about the chlorhexidine. Often hard to give advice to people close to us, though!
Also, I wonder if your friend is expecting little kids to brush/floss well themselves? I feel like it's necessary to help them--usually doing it for them--until they really get a good hang of it. Unlike me, my 3 kids have never had a cavity.