Step one: “hang on a second, folks; Aditya, you just played Teferi, right? … OK, Ned, do you recall which lands were tapped to pay for that? you know, that's six mana, right? and Teferi only costs 3UW?” all while observing Ned and even Aditya for their reactions; this gives you an early read on the possibility that Ned just wasn't going to correct an error they noticed.
From there, it's pretty easy to get Aditya to tap correctly, confirm with …
Step one: “hang on a second, folks; Aditya, you just played Teferi, right? … OK, Ned, do you recall which lands were tapped to pay for that? you know, that's six mana, right? and Teferi only costs 3UW?” all while observing Ned and even Aditya for their reactions; this gives you an early read on the possibility that Ned just wasn't going to correct an error they noticed.
From there, it's pretty easy to get Aditya to tap correctly, confirm with Ned that all is OK, and let them keep playing. (The most likely outcome; if your “spidey-sense” tingles in step one, you may pursue a chat with Ned, see what's in each player's hand to help you understand possible advantages, lines of play, etc.)
Step two: OK, keep playing - sorry for the interruption. Sam, can I speak with you, please? … (away from match) “Hey, at Competitive REL, if you see a problem, it's OK to ask the players to pause while you get a judge, and tell them - AWAY FROM THE TABLE - what you think went wrong. Please let Judges handle the actual infractions and any remedies. Oh, and - at Professional REL? you can't even stop the match, just get a judge ASAP.”
No infractions (in almost every reasonably conceivable case), no real harm done, good intentions all around.
d:^D
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