I haven't tried essential oil of grapefruit for bad breath. Grapefruit is known to be cleansing. It's not listed in my reference books under "halitosis". Pretty well all of the essential oils are anti-bacterial, so it may help. The important thing is to make sure that you're using therapeutic grade essential oils, not poor quality ones or ones that have been watered down, etc.
I also sometimes use Thieves spray as a breath freshener. It works. You have to like spices though.
Unfortunately, I can't seem to keep house plants alive for long, but I agree Spud, mint leaves are a great idea too :)