Facts are missing and it could indeed be ketosis that created the suspicion.
Regarding some of the comments on here, I am a big believer in giving a pilot a chance to get well through a comprehensive treatment program like the one ALPA administers. However, the notion of quietly sending a colleague off the aircraft because they’re under the influence is dangerous, IMO. Anyone who’d take it all the way to getting on board is demonstrating a wreckless disregard for safety and deserves the embarrassment of an arrest. Maybe, just maybe, that act would be the rock bottom wake up call they need to get help.