Short explanation, because it's probably important to my question: Not too long ago I left my previous place of employment, let's call it Company A. I was working as department lead, and I left due to some minor conflicts with new management, and general mismanagement of company, but we parted on good terms.
But my departure had some consequences in the next several months, although not direct. In short, it led to problems in my department, which led to problems in departments that were working together with us, which in turn led to several other important employees leaving. Obviously, all this led to problems with projects and client.
Also, shortly after my departure one of the major clients of company A contacted me and offered me to work for his company as more or less independent contractor, and I agreed. But recently we got a rather big and important project with very short deadline, so I won't have enough time to assemble new team or finish it on my own. So my client decided to contact my previous employer. They didn't seem happy to work with me again, but they agreed nonetheless. And here is where my problem lies.
On the one hand, I always feel responsible for my projects and for the success of clients I work with. And, as an "insider" from company A, I know about some of the less ethical of their practices. For example, presenting less experienced employees to clients as more experienced, or people who work on 2-3 projects at the same time as "full-time contractors". These practices led to several failed project while I was working on company A, and were part of the reasons why I left. So I feel that my current client should know about those practices, to ensure success of our current project. At the same time, it really feels like badmouthing my previous employer, and it also doesn't sit well with me. So, what do you think would be ethical or right thing to do in my current situation?
Small edit: waiting for my original company to screw up, so their problems will become apparent isn't really an option. By that time it may be too late to salvage the project, if we will need to find new contractors, transfer work, project information, etc.