Can I tell my boss that my colleague is doing nothing?
Why would you say something like that to your boss?
I have no respect at all for people who don't do anything
Talking directly to the person is preferable, since that gives them a chance to correct the problem without getting in trouble. Only go to your boss as a last resort.
Nope, I'm going to take care of this now.
Why would you not have solidarity with your coworkers? You work with them and you shouldn't go around telling on them unless what they're doing affects you.