A.,
That would be a difficult situation for me to tackle as well. However, he is a grown man, and although his choice does not appear to be the most informed, there is really so much you can do to encourage him to do things differently. Sometimes the more we push, the more resistance we get. If you were to only brush your teeth once in the day, the best time is in the evening because of all the bacteria that sits on your teeth over night if you don't brush. Some options he has if he is not going to brush would be to chew gum with Xylitol (Ice Breakers Ice Cubes) a lot of research has been done to show that xylitol helps reduce the risk of caries if chewed consistently, especially after meals.Drinking water throughout the day may also be a good choice. Also, he may not be used to what really clean teeth feel like, and therefore be able to tolerate not so clean teeth. As far as oral health, if he is not a snacker, doesn't drink much soda or acidic drinks (am coffee the exception) throughout the day he may be better off than most as these things lead to decreased oral health. If he visits the dentist, they will be able to see the care he is putting towards his oral health, but even then, dentists can give suggestions, but they can't control what people do on their own. Some dentists will not work on people who don't take the time to take care of things once suggested, that is usually in extreme cases, but you can limit the options available to you from your dentist by not doing your part. Sadly, there are a lot of people out there with pretty gross mouths.
I suppose your biggest problem is that he is a grown-up and he chooses this for himself. There is really so much you can do, and you have to decide how this weighs in on your relationship. Is is a deal breaker? Will you be stuck on this and will it interfere with the friendship/relationship you have built with him over the years? What if he does nothing to change this habit?