If you think this is an action worthy of calling it out "snitching," then you know what the friend was doing was wrong. If you don't want to be snitched on, don't blatantly do something illegal in front of others showing it off. Sounds to me the friend was almost blatantly flaunting it considering when and how it happened.
Do you whatever you wish to do OP. If your friend is going to get angry at you for reporting their gil buying, then THEY'RE not the real friend here. It's a game, yeesh.
