My Dad's tax preparer (who used turbotax online) has a stroke. Is there a way that I and my Dad can access his 2017 tax file?

First a paid tax preparer could NOT have used Turbotax since it is not licensed for professionals. 

Now if they did use the online version to "help you to file your own return "   then you have to have the userID & password to access the file.   

How do I get a copy of a return I filed using online TurboTax?

https://ttlc.intuit.com/questions/1900748

How do I get a copy of a return I prepared using desktop TurboTax?

https://ttlc.intuit.com/questions/1900639

You can get a return transcript from the IRS for free :  http://www.irs.gov/Individuals/Get-Transcript

For a $50 fee you can get a copy of the return from the IRS:  https://ttlc.intuit.com/questions/1901218-can-i-get-a-copy-of-my-tax-return-from-the-irs

Paid tax preparers cannot use the online TurboTax program.  It would violate the terms of service.  If he was a paid professional he would have to use the professional software, which you cannot access.  Even if it was just a family friend who did your father a favor and prepared his return unless you know the account and user ID he used, you cannot access the account.

You can get a free transcript from the IRS, or for a fee of $50, you can get an exact copy of the return from the IRS.

You can get a free transcript from the IRS:   

https://www.irs.gov/individuals/get-transcript