Why don't you ask him the account name? Search for it, or have your friend add you first.
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Once you have his account name, search it. If his stuff doesn't appear in the search results, then ask him what friends he has, then you can search for them and find him through their friends. If that still doesn't work, ask him to comment on a featured project, then find his comment and click on his name. Hope this helps, and if you have to do the comment thing, make sure it's not spam.
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You can't find him in the search results because he doesn't have a project. Ask him to make a project so you can search him up, or you make a project so he can find you.
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If you know his username, you can view his page by going to:
http://www.scratch.mit.edu/users/username
Except you'd change the "username" bit to their username, obviously
From there, you can just click the "add to friends" button
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