Hello,
I get a 'feeling' that the person in the posting COULD be YOU--anyway, that wouldn't affect/influence the suggestion(s) of the members.
Coming to the options available to the person, First and foremost, suggest him to have all the discussions/decisions taken along the way from now on IN WRITING. I know it's tough to do it without raising suspicions among the management. One way--that worked out well for me in the past--is this: After a verbal/oral discussion/meeting where any decision(s) is/are taken--especially if this person gets a feeling that something CAN go wrong--suggest him to send a formal/innocuous email to those who attended the meeting. It could be in the garb of Minutes of meeting or just a Summary of what was discussed. The logic/reason for such a step is two-fold: (1) to put things in writing and safeguard himself for any later unpleasant situations AND (2) to ensure that all the participants are on the same page--this would be the positive aspect. He can end the email/note with a line saying/asking others to correct/add/modify if he has missed out or misunderstood any points. This would ensure that the general agreement among the participants is on record, even if no one replies to his email.
Hope you got the point.
Coming to his being scared of the company's past history of vindictive attitude, going by what you mentioned, I have a feeling that this would be known even outside the company--among the competitors and companies in a similar line of business. So any bad-mouthing MAY NOT have much effect--if that's what he is fearing--ASSUMING this person has a reasonably good reputation outside the company circles.
Coming to another aspect, you haven't mentioned the perceptions among the clients about the ACTUAL reasons for the situations which forced them to initiate action. I would differentiate the 'action taken' from the 'perception' of the clients--the former being what they are expected to do officially and the latter being 'in the know' of the issues 'within the company' that led to the situations.
If the clients DO KNOW about the internal issues, then I think this person can take some of the clients as references, INSTEAD of the company references, when he is looking to change.
Regards,
TS