The informant’s grandfather always told him this when eggs were served at the breakfast table—don’t drink water right after you eat eggs.  Why?  Because, the grandfather said, it will give you really bad breath.  You have to at least eat bread in between, but drinking water right away is a recipe for disaster.

The informant says this is something his grandfather has often said at the breakfast table.  The informant said he doesn’t really believe it, but still remembers his grandfather’s words whenever he eats eggs.

I agree with this interpretation.  I would suggest that perhaps, one time, the grandfather or an ancestor of his incidentally was stricken with terrible breath after eating eggs and drinking water, and this was where this folk belief came from.