It is a gamble, but I think I'd be fairly sanguine about the odds. None of the suicidal folks I've been close to (and there have been a few) have given me the impression that they were making an unbiased cost-benefit analysis of their futures, or indeed were capable of making such an analysis; extreme depression taints expectations badly, and so do most of the other problems that can lead to suicidal ideation. Get them to commit far enough in the future that their troubles are likely to ease, and I'd say it'd be more than likely that they'd drop the original plan. Especially if you can then get them to commit to therapy, exercise, or another of the common strategies for building emotional stability in the meantime as a palliative measure.

And if they are making that cost-benefit analysis, perhaps the horse will learn to sing.