Cervix73 has it spot on. Just done some command assessments and that was one of the questions I prepared for.
Give the guy/gal a way out, tell them you suspect they've been at the booze and tell them to go sick.
Then tell the rest of the crew that they're sick. Phone crewing tell them your colleague has gone sick and that you need a replacement. Once you've got a few spare moments (suggest a good few hours later) phone the flight ops manager with your thoughts.
That way you're covered, your colleague can't be fired because their isn't any proof and he knows he's going to be watched in future and has a chance to admit a problem which can then get sorted.
As has been said previously if someone turns up in that state there's probably a background situation requiring attention. I think most employers would rather get the person sorted and on the straight and level rather than have a nasty incident/exposure on the telly etc.