Had this issue before, and they need to become friends right after the break-up. "Ask to be friends". My sim didn't do this, instead she bought the business and fired her husband (his idea to break-up, so I, not her, was mad) and everytime she sees him she wants to 'fight". Now, he is sending her love letters and each time she sees him she wants to "fight".
So, she sent him a love letter and the response: "I find you cute and all; but, I'd rather tear out my eyebrows with a plumber's wrench". So much for reconciliation!