It is a requirement (at least in my company and I believe the CAA-Etihad's regulators may have similar rules, most authorities are the same) that a person is not drunk when sitting at an emergency exit.
If the CC felt(or in any doubt) that person had already enjoyed a few preflight, it may of been a sensible precaution. I have sat in the emergency exit of a Etihad flight previously and been served alcohol.