So, how many wins have you agreed to him for the duel? I think if you're duelling someone for something else than Toricredits you have to make a make a deal on how many wins should any of you get to win the duel.

If it's only 1 win then you better PM him first before reporting him since he said "g2g" after lost the duel.
If it's more than 1 and then he said "g2g" after you won the match, I think the duel is posponed and maybe he'll continue after he done with his irl stuff. Try to PM him to make a deal about when he can be online and continue the duel.

Wait for 24 hours atleast, if he hasn't come back, you can report him with the evidence you got.