Family member’s business hacked. Need help.

My sister owns a company and has employees on payroll. Last night a hacker got into her payroll account and tried to send themselves $150,000. Luckily the payroll company clocked it and locked her account but the transfer has already initiated. She contacted her bank to freeze her account, so far she still has the funds in there. The payroll company suspects someone has access to her laptop because the login came from her laptop at 5 am this morning. They also probably have access to her email because they were able to receive the MFA code. Right now we have the laptop powered off and are deciding what to do next. I want to reinstall Windows from a usb drive, but want to know if that’s enough to remove the hacker? Are there any other precautions we should take?

Edit: she has been advised already to file a police and fbi report and will be doing so