Many ways you can play it.
The first way is the least damaging to the person concerned. See if you can persuade him/her that they're not well and suggest that they go sick. See if you can find out what's on their breath. eg it could be ultra strong mouth wash. Unlikely I know but it pays to err on that side initially.
Ease the "pain" so to speak of the person concerned and tell them that you'll call crewing/despatch and arrange the hotac. Basicly make it easy for the crew member to go sick if you know what I mean.
If it's a one off, hopefully those actions should sort the person out and they'll feel suitably chastened to realise how close they came to doing something really stupid. Best done before they've signed on.
If it's part of a trend for this person then mention it to a colleague that you both can trust.