If they cannot grasp that simple logic, I'm afraid there's not much point in arguing the matter further with them.
Even if you beat them over the head with a book of irrefutable quotations, they still would attribute it to some Vast Right Wing Conspiracy to "rob" Mandela of his glory. (That's just how Liberals think.)
The real problem is that when you search the interent there are abundant sources claiming both. So I need to find something that predates Mandela's arrival on the scene as a Marxist hero.
There's loads of dumb crap on the Web. Remember that idiot politician who claimed that Flight 800 was shot down because he found "proof on the Internet"? That's the sort of brain thrust you're dealing with here.
Anyway, I think they'll have a hard time contradicting Bartlett's. That is perhaps the foremost authority on confirmed quotes attributed to the correct party.
And lo...it is in Bartlett's Familiar Quotations.
Tell 'em to buy their own damn book and to stop being such pig-headed Liberal fools.
http://www.amazon.com/exec/obidos/tg/detail/-/0316084603/ref=ase_toliveistoplay/102-7708260-7847330?v=glance&s=books