This is not deductible on your personal tax return.
Is the employer's plan a SARSEP (Salary Reduction Simplified Employee Pension) established before 1997? If not the only SEP contributions permitted are employer contributions which are deductible on your employer's tax return, not on your personal tax return, because your employer does not include this contribution as part of your taxable wages. If it is a SARSEP and you made elective deferrals, the employer excludes the elective deferrals from your taxable wages shown in box 1 of your W-2. Since you are not getting taxed on this money to begin with, you can't deduct it on your tax return. Enter any amounts shown in box 12 of your W-2 only in box 12 of TurboTax's W-2 form, nowhere else in TurboTax.
"25% of income" suggests that this is not a self-employed retirement contribution. A self-employed individual would be permitted a maximum SEP contribution of only 20% of the result of net profit minus the deductible portion of self-employment taxes. It sounds to me as though this individual is thinking of the SEP contribution as part of compensation when it's not, but I could be wrong.