How can I go back to Office 2007 after my computer automatically updated to Office 2010?
There are many ways to have Office 2007 on your PC, and many ways to install Office 2010 (though as Jan points out, this is not done automatically as 2010 is a different product). Therefore, it's not possible to tell you exactly how to "revert" to the old version without knowing how you got the software in the first place. for example, both versions of the product may still be installed on your PC. You may have removed a "bundled" version to install the new one (in which case, the only way to get the old one back is to reset the computer to factory settings), or you may have some volume licence that includes downgrade rights.
In short, give us some more background and we may be able to help you. Without that, all you're getting is stabs in the dark.
Upgrade options and returns:
If you purchased an otherwise returnable product with an upgrade option, returns will be honored within the return period, and special return procedures apply as follows:
-If you return the product before the upgrade is released, you will not receive the upgrade.
-If you return the product after the upgrade is released, you must return both the product and upgrade together.
OEM Office licenses do not have downgrade rights to previous versions.
Volume Licenses offer downgrade rights to previous versions.
According to Microsoft:
Downgrade rights in Volume Licensing programs give you the right to downgrade to any prior version of the same product. Therefore, you may downgrade Microsoft Office Professional Plus 2013 to the Microsoft Office Professional Plus 2010, Office Professional Plus 2007, Office Professional Enterprise 2003, Office Professional XP, Office Professional 2000, Office Professional 97, Office Professional 95, or Office Professional 4.3 versions. You may not, however, downgrade to Microsoft Office Standard Edition 2010 because it is a different product and not considered a prior version of Microsoft Office Professional Plus 2013.
In addition, Office Professional Plus 2010 users may not use earlier versions of the Microsoft Office OneNote note-taking program and Microsoft Office Groove software via downgrade rights, because these products are not part of October 2013 earlier versions of the suite (for example, OneNote 2007 and Groove 2007 are not included with Office Professional Plus 2007).
How to Downgrade to Office 2007
if you have your Office 2007 license then uninstall Office 2010 and reinstall Office 2007, now perhaps you will need activate via the phone option.
Maybe he simply likes Office 2007 better than Office 2010 so he wants to revert after manually upgrading. It is like many people wanted to revert their OS from Vista back to XP although they paid to purchase Vista, and I don't understand why it is necessary to ask him "why". Just teach him how to do it if you know...
How did your computer "automatically" upgrade to Office 2010?
An Office 2007 license is not valid for Office 2010. In other words, upgrade costs are involved which means it would/should never perform an upgrade without someones interaction.
Also such an upgrade is not offered by any automatic means like Microsoft Update or similar. To the best of my knowledge it would require a manual download e.g. via the Microsoft Store or an actual installation CD.
The only exception I could think of is for computer within a domain where the upgrade is issued via Group Policies. In that case you should ask the administrator about downgrading.