Search for your colleague's name and see if they have a Citations profile. If they do, click on it, and click the "Follow new articles" link in the right sidebar under the search box.

If they don't have a profile, run a search by author, e.g., [author:s-hawking], and click on the envelope in the left sidebar of the results page. If several people share the same name, you may need to add co-author names or topical keywords to limit results to the author you wish to follow.