We all know how Indians came up with that word. It is derogatory for most Nepalis who're not working for them. It's like white people using the "n" word in the USA. Nigro means black in spanish , so basically white people are calling blacks as black but the term has history of steriotyping, racism and slavary attached to it, so it is aderogatory term now. Same thing with "bahadur" used by indians to Nepalis. It is not just limited to being brave, but it means we're their servants doing menal jobs. It symbolizes steriotyping when Indians use it for Nepalis. Good job on rubbing it on his face. If i were you, i would probably say to everyone that this word is steriotypical and very offensive like using N word in the usa for blacks and i'm very offended,  and make him look like a jackarse /racist infront of others. but i like your approach very much.