Who writes off the amount put into a SEP, the business or the employee?

Answer 1

The employer deducts contributions made with the employer's money. 

The employee's taxable income is reduced by the amount of contribution that comes from their own wages/salary. 

Answer 2

Who contributed the money? If the business paid it without deducting it from the employee's paycheck, the business deducts it as an employee benefit. If it was a post tax deduction from the employee's paycheck, the employee deducts it.