It seems to me that this could be interpreted to allow the following scenario: A police informant runs out of gas in front of your house. You let him in to use your phone so he can get a ride. The police then mysteriously show up wanting in. You tell them no but from behind you the informant yells "come right in."
That's not what's going on in this case though.
Using your case as an example, you kindly let the informant in. Later, police come to your door. The officer asks "may we search your place?" You say "no". Doesn't matter what the informant says. Your "no" still rules, as long as you are still there. That's still going to be the case.
US v. Matlock, 1974 allowed the search as long as someone who could consent did consent. "Government must show, inter alia, not only that it reasonably appeared to the officers that the person had authority to consent, but also that the person had actual authority to permit the search..."
Georgia v. Randolph, 2006, changed it so that if any occupant objected, then the search could not take place.
Today's ruling, Fernandez v. California clarified and limited the exception from Georgia v. Randolph. If the person who objected to the search isn't there, and the person there is able to and does consent to a search, the search is valid.