Pfff, much luck to him for ever figuring it out with those two! xD

Hmm, it'd depend? He wouldn't do anything overly aggressive or suspicious while they're in open public, but mind games are always fair game as far as he's concerned. If he were directly involved he'd take great care to be an useful aid to Yuugi about figuring out how to transform, just like how he was so very helpful during the duel against Otogi. If he only happens upon the scene late he might try to pick him up and ask about Yuugi's ideas on partners, but not do anything outright worse either (except maybe planting a few ideas and doubts; he knows that he's on his suspicious-people-list still).