As long as you did not lie (so that you committed defamation or filed a false report)  or in some other fashion misused the official proccess (e.g. called the police purely out of retaliation, when the fellow's condition did not warrant police intervention), there should be no cause of actionl. Someone is not liable for calling the police when there was a good faith belief of a crime and/or threat to safety. Assuming the individual was in such a state as to justify contacting the police, such as before he drove, you should be in good shape. That said, it's hard to stop someone from at least suing if they are determined to, and all suits need to be taken seriously. If sued, retain an attorney to help defend you--there's a good chance he or she can dispose of the suit fairly quickly--and also consider whether there are any countersuits, such as for filing frivolous litigation designed to harass, which you could bring in turn. (At the least, you may be able to get your attorney's fees.)