Brewing coffee the night before is perfectly normal, but leaving it in a thermos for several days is not the best way to store it. A thermos is designed to keep drinks hot or cold for a limited period, not to preserve them safely for days. Once brewed coffee gradually cools into the temperature range where microorganisms can grow more easily, its freshness and safety become less predictable. Even plain black coffee can pick up bacteria from the container, lid, hands, or surrounding environment. After three days sitting in the same thermos, I wouldn’t recommend drinking it.
If the goal is iced coffee, a better method is to brew the coffee, let it cool without leaving it at room temperature for hours, then transfer it to a clean, covered container in the refrigerator. Plain refrigerated brewed coffee will generally keep for several days, although its flavor starts deteriorating fairly quickly. Around 3–4 days refrigerated is a sensible limit for homemade coffee if it has been handled cleanly. Even before then, oxidation can make it taste stale, bitter, or flat.
Coffee containing milk, cream, or other dairy products requires more caution. Once dairy is added, the drink should remain refrigerated or be consumed promptly. Leaving milky coffee at room temperature for hours—and especially overnight—is not recommended. Sugar and flavored syrups can also change how the drink ages, so preparing plain coffee first and adding milk immediately before drinking is usually the better approach.
So your partner’s idea of preparing coffee ahead isn’t necessarily a problem—the storage method is. Brew it the night before, refrigerate it in a clean sealed container, and pour it over ice when needed. But keeping the same batch in a thermos for three days is something I’d skip. When coffee develops an unusual smell, visible film, mold, or strange taste, throw it away rather than trying to rescue it.
