Even though you've changed your password this person would still be able to login to the account unless you've went to this site here and clicked the "Require Profile Download". That way when the person goes to login to the account again they will be asked to enter in the new changed password. You'd also want to go through and change the e-mail associated with your gamertag as well as change the password for your e-mail account (just in case).
To learn how to change the e-mail associated with your Microsoft Account you can visit this page here.
You will also want to take a look at the security checklist here. I also suggest you take a look at your billing and payment information over the next couple of days to make sure nothing was charged that you didn't personally authorize, you can do that by logging into the Microsoft Billing site here. If you do notice that there is something that you did not charge yourself on the account please contact XBOX Support via phone by clicking this link here.
If you have any further questions, please let us know :).