1. Do not misrepresent your company.
Important part of the interview is when you ask the candidate about his expectations of the new job and give him the overall idea about the work processes in your company. One of the points here is to make sure that your company doesn't, by any chance, have the same traits that were getting on candidate's nerves at his last workplace. Because if it does - the candidate would find that out the day he joins in, and he would certainly be unhappy about not knowing that up front. While you (and your team, and your company) want a happy colleague who wouldn't think that he's been tricked. Hence, put the facts plain and square, neither concealing them, nor decorating nor disfiguring your company. It is the right thing on many levels: making sure that the prospective candidate would stay; maintaining honest company profile; identifying weak points of your company and bringing them to the management.
2. Protective advices are for close friends.
To begin with, taking a personal advice requires trust. Are you sure he trusts you enough to take such advice from an interviewer?
The second reason is that once he has all the facts he would make his own conclusions and wouldn't need that advice anyway.
3. Let him have his opinion and be his own master.
People who has left (or are going to leave) often tend to paint the things more grim than they appear to an uninvolved person. It's not about being dishonest, it's more about accumulated dissatisfaction. His opinion may vary. If he thinks that those facts are a molehill - so be it. Perhaps, he values that particular job higher then possible long hours. If after a few years he changes his mind - so be it, too.
4. Speak only for yourself.
Don't speak for other people. Statement like "we all think that ..." sounds like you're striving to give your claim all the weight it may get, while at the same time excusing yourself of sole responsibility for such opinion. Sorry, that just wouldn't smell good.
At the current stage, when your communication with the candidate is over, I wouldn't advice you to contact him behind the company's back. The best thing (in terms of honour) I can think of is to persuade the HR person to give the prospective candidate the key facts about the job. On the ground of representing the company in a fair way and hiring a consciously willing candidate.