No, you cannot amend the 2018 tax return.
If there is FICA tax withholding shown on her 2018 W-2, you will have to report it on the 2018 tax return so to match the IRS records. You cannot just amend to change the information on the tax return unless her employer went back and adjusted the information on her 2018 W-2.
Since it appears that her employer might have reported her taxes incorrectly on her W-2 ( that F-visa OPT holder is not subject to FICA), she would need to contact her employer to resolve the issue. If she cannot obtain the FICA (Social security and Medicare taxes) from her employer, you can file a Form 843 "Claim for Refund and Request for Abatement" to request a refund from IRS.
To file a Form 843, click here https://www.irs.gov/forms-pubs/about-form-843
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