they won't be able to give up unless they really want to. they might know they have a problem (it seems they do from the fact they have cut down), but they might not be ready to stop. That's a fantastic step in the right direction, though.
I guess gradually cutting down bit by bit would be the way to go. Spend time around people without doing it to realise you can have a good time without it.
They should definitely avoid being around other people who do it as it's too easy to join in. Even if they're with other people and haven't got any all it takes is for someone to mention it...they all get thinking about it..and one call is all it takes.
Even if they 'give up', they may find on the odd occasion that they laspe. This happens to many people with addictions. But rather than getting cross, the best thing is to brush it aside, put it down as a mistake and start with a clean slate.