You have a number of options.
You might be able to take control of his account by calling customer support. You will probably have to provide some kind of documentation that you are authorized to file his final return.
It's not necessary to take control of his account if you have a copy of his 2024 return. You can create a new account and file his 2025 return, any information that might need to carry over will be in the 2024 return. If you don't have a copy, you might be able to get a transcript from the IRS, or again from Turbotax by calling customer support and supplying some proof.
However, be aware that each turbotax online account can only file one return for one person, and each return will have a separate fee, unless it is a simple return that qualifies for free filing. If you need to file more than one return (such as for yourself or other family members) it is probably more cost effective to buy Turbotax as a download to install on your own computer (the desktop version), especially if you use a discount retailer like Staples or Amazon. Using the desktop program you can prepare multiple returns, and the only extra fees are state e-filing. To prepare your father's 2025 return using the desktop program, you again may need a copy of his 2024 return (if there is anything to carry forward), which goes back to the paragraph above about how to get it if you don't already have a copy.
