In absence of war, the secession would have become more solid with each passing year as it gained acceptance on both sides."
Sorry, but that's exactly backwards -- Lincoln did not "need" war, did not want it, and did not start it, regardless of how mis-educated your friend is.
The person who needed, wanted and started Civil War was Jefferson Davis, and for reasons which should be obvious, and have been explained on this thread now several times.
- Davis' rump-Confederacy of just seven Deep South states was not, by itself, viable economically, politically or militarily.
In a nation of 31 million people in 41 states & territories, the rump-Confederacy of seven states and 2.5 million white citizens was outnumbered more that ten-to-one.
They were also nearly outnumbered by their own slaves. - Economically, the rump-Confederacy had only one major product to sell, cotton, along with some tobacco and sugar, no manufacturing to speak of, and absolute dependence on foreign customers, ships and bankers.
Yes, they had significant railroads, but those were a mish-mash of gages not always interconnected, and barely extending west of the Mississippi. - Yes, the rump-Confederacy was prosperous, very prosperous, arguably the most prosperous region for white settlers in the world.
But this prosperity was 100% dependent on two factors: ever increasing international demand & prices for cotton and ever rising values of the slaves which produced it.
These increasing asset values allowed farmers and plantation owners to borrow from international banks far beyond what their current incomes might allow, and made them extraordinarily vulnerable to any disruptions in the market-place.
Of course, Confederates chose (unwisely) to look at it from another angle: if they stopped the flow of cotton to Britain & France, they expected it would increase cotton prices and force those countries to ally with Confederates against the United States... - Militarily, the rump-Confederacy could soon be overwhelmed by much larger Union Army and Naval forces.
- Pro-Unionist sentiment, even in the rump-Confederacy would never be entirely stamped out, so long as they remained at peace.
- Most important, Virginia and the Upper South had already voted not to join the rump-Confederacy, until or unless there was some act of "oppression" by the Union, and in practical terms, that meant there must be war.
War would, at a stroke, at least double the Confederacy's population with four more states, plus add significant industrial war-production capacity in Virginia and Tennessee.
And war would make slave-owning Border States (Maryland, Kentucky, Missouri) contestable by Confederate forces. - In the experience of senior Confederate leaders like Jefferson Davis, "Dough-faced" Northerners were unfit to make good soldiers or leaders of Armies, and so a stout show of Southern manliness would quickly scatter them from any battlefield.
- War was also opposed by many Northern Democrats, and some "moderate" Republicans, and several sharp defeats could well force Lincoln's cabinet (i.e., Sewart) to seek a negotiated settlement with Jefferson Davis.
In short, for Davis starting a war at Fort Sumter was a no-brainer -- absolutely necessary, and only likely to lead to a greater Confederacy, military victories, and long-term preservation of their "peculiar institution".
And, if you think about it, Jefferson Davis' calculations were 100% correct, except for one (using some modern phraseology): Davis didn't know him, and mis-underestimated the stubborn strategery of the decision-maker in the White House, "Ape" Lincoln.
Southern railroads in 1861: