I just changed my password, but one time I used my friend's mobile phone to access my account using the add account feature. Sometimes she uses my account to write or read something, and she still accesses my account from her phone.
Can I log myself out of all instances of the account?
Try logging out of your Twitter account after logging in, it might logout your account from any device it's logged in on.
But on most mobile devices, apps login to your account and to make the app logout from your account. To revoke access of apps go to this URL https://twitter.com/settings/applications and Revoke Access to the app your friend's device uses for Twitter login and your friend won't be able to log into your account again without your permission.
You can also change the password and it will log out of all accounts.
