Hello,
Did you remove the friend’s account from Settings, Accounts, Family and other users, or only from Email and accounts?
In Windows 10 Home, the sign in screen shows local user profiles saved on the device. If the profile still exists, the name stays on the startup screen. Open Settings, Accounts, Family and other users, select the friend’s account, then choose Remove. Restart the PC after removal so Windows refreshes the sign in screen. The startup list comes from user profiles stored on the device, so removing the profile removes the account from startup.