Question about DUI laws in the state of Missouri?

Can an officer charge you with a dui even if you aren’t operating a vehicle, nor even in a vehicle? Can they charge you by suspicion of drunk driving? My friend was suspected of driving drunk, and was given a breathalizer, and failed it. They arrest him and charged him with a dui even though he wasn’t even in a vehicle. Is this legal? If not, please give me a link to prove so.