I'm sorry, but you'll need the 2018 TurboTax software (same edition your client used) installed on a computer to open/read the tax data file. You can purchase this software through TurboTax.com, and possibly (?) through a reputable online retailer (like Amazon). I'm sure you are already aware that the time frame for amending a return in order to receive a refund for 2018 has lapsed.
Here's the link to the TurboTax website where you should be able to purchase the 2018 software.
TurboTax® 2018 Tax Software for Filing Past Years’ Taxes, Prior Year Tax Preparation (intuit.com)
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***
