I haven't examined the statistics myself, but isn't it known that suicides go up when there are news stories about them, even in a preventive effor? This implies that suicides are, in fact, somewhat suggestible.
On a personal level, I've found that talking frankly about suicide greatly reduces my desire to act on any suicidal feelings. This effect is greatest when talking face to face with somebody I know and trust.
However, worrying about whether or not somebody trusts you should NOT stop you from intervening. If somebody shows a desire to listen and help my in such a situation I'll have a much greater level of trust in them than I did before, even if they were previously a complete stranger or somebody I dislike.