While your intent to pass on the feedback you received to the client is quite laudable, please also bear in mind the other serious consequences that may arise.
Frankly, the feedback you received from your candidates is something that you ought to have done before placing them in this company—checking the general culture, ethics, and environment before accepting it as a client. In a way, haven't you put your candidates in trouble? Please give this aspect some thought.
Considerations Before Informing the Client
Coming to your proposed action of informing the client, please consider the following points/aspects:
- Since you haven't mentioned how many candidates you placed in this company, one aspect is tough to judge. If the number isn't high (say, in single digits), then what do you think the company HR could do? Is there any possibility of HR checking out who, among your candidates, passed on such info to you? And if they did, are you sure this won't go against your candidates? If the number of candidates placed is higher, it could be tough for HR to figure out this aspect, but it could lead to pressure on you to reveal the source. Can you take it if it happens?
- If you are absolutely sure, then what you wish to do could be good/right. But even if there's a 1% chance of your candidates taking the flak, then I suggest dropping the idea of giving the inputs directly to the company (if you have to give the feedback to them)—look for other ways to convey the same information without jeopardizing your candidates' future. And even then, only if you feel the information you provide will be rightly/correctly utilized by the company. If not, your good intent could be misunderstood.
Format for Providing Feedback
There's no specific form/format to give such inputs/feedback, to the extent I know of. However, even if any format does exist, I would suggest you refrain from putting such things in writing unless absolutely necessary. After all, it's the inputs you give that need to be the focus for the company—not how you give them—if they are genuinely serious about correcting things at their end.
Hope you understand the point.
Best regards,
TS