So many Lawful Neutral and Lawful Evil people here. .-. Since he's my friend (And he's my friend, it is for a reason), I wouldn't report him. He's doing that on his own and, any consequence beyond that it's his responsability alone. If something happens, the first person he'll point out will be you.

I wouldn't even compare buying gil with thievery, it's on a different spectrum. He's not abusing on other people. He's just one gear in this huge system. You can talk to him and tell him it's wrong to do that. Don't backstab him just like that.