That's small consolation to him; his brother, former president of Nigeria, died last year leaving several million dollars behind. They are currently trying to recover them through the aid of western citizens.
Note that the two statements are quite different. The guy you replied to was implying that all Nigerians are scammers. I, on the other hand, implied no such a thing and simply made a joke about a type of fraud commonly originating from Nigeria.
I get the joke. But it's not funny. I've read the same joke about 1000 times on various forums. It's just a somewhat mean statement with no humour at all.
Millions of Nigerians work honest jobs to feed and educate their children. It's hard enough that they have to live through the negative stereotype caused by less than 0.1% who are bad guys.