I was going through a friend's tagged pictures and accidentally tagged another of our mutual friends in it. How do I remove the tag? (The picture was uploaded by someone who isn't a friend.)
There's no option to remove a tag of your friend you requested on others photo but however you can remove your own tag.
If you tag yourself or your friend in somebody else's album, they are sent a request to accept the tag and its on them to accept or the reject the tag.
And if you want the tag to be removed, try to message the person who uploaded the photo and ask him to remove the tag. That's all you can do on your side.