Etiquette for reaching out to previous students of advisor
I can only ask myself how I (a PhD scientist in industry in the US) would react upon receiving such a mail from one of my former advisor's current students. 


If the mail asked for general career or technical advice, I would be happy to respond. In this case, there is really no need to ask the scientist to avoid mentioning the contact to your advisor -- (a) I probably wouldn't anyway, and (b) if your advisor would be upset by this, then you have bigger problems. 
If the mail was asking for advice in dealing with the advisor, I would really look at how it was written. Tact and concision here goes a long way. For example, "Bob has been shouting at me 3-4 times a week and I am not sure how to proceed" is something I would want to advise you about; many details about the complicated lab politics are something I would prefer to avoid wading into. In this case, adding a brief note requesting that I don't broach the issue with your advisor directly will not do any harm (though, there is always a 1% chance that the mail somehow ends up in front of your advisor -- which is all the more reason to be tactful).
If the mail were asking for a brief phone call, I am less likely to respond (I hate phone calls and love e-mail; YMMV). But, if you simply must share things that absolutely cannot end up in front of the advisor, a phone call is your best bet (even if the advisor finds about the phone call, at least he won't have the transcript; this gives you a lot more flexibility).