Seems like your friend is following some very convoluted logic. First off, if the South was winning, why would they sue for peace? And if the North's purpose in fighting the war was to "free the slaves," wouldn't it make more sense for them to approach the South with the proposal that if the slaves were freed, the North would discontinue their ongoing invasion and allow peace to reign?

At any rate, it doesn't sound like you're going to convince this guy of anything by using a logical argument, or facts, or anything like that. Good luck.