Don’t Draw Conclusions too Quickly

Ansel is playing against Nigel in a Modern PPTQ. Ansel plays a Mountain, then plays and attacks with a Goblin Guide and says “trigger, then its your go”, Nigel adjusts his live total, then reveals a Fiery Conclusion and then puts it into his hand. Nigel untaps his only land, then draws a card. Both players immediately realize the error and call for a judge, and explain exactly what happened.

What do you do?

Answer
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