"Just as you wouldn't tell your doctor how many nurses and anaestheiologists to bring into the operating theatre, you have to trust your professional services contractor to make the call, within the remit of their contract, to use budget and their own staff in the manner they judge most effective."
You're kidding right? If I was a client I'd want to screen every CV and know exactly what people would be doing on a project before they came. We all know that all too often a project is staffed by whoever is sitting around the office whether they know anything about the project topic at all. Worse, the reason they're sitting around is that most often they're the worst people.
Telling clients to "relax" that us consultants would never dream of stitching them up is madness!
In the case of an experienced resource being free, take it and be happy though. When they start pointing out how useful they are and saying maybe they should get involved just say you won't move on budget, but it'd be great if he could remain part of the team. If they're having a quiet time (which can be the only reason for this) then you may just get them free for ages. During the early 00s downturn some consultancies sometimes had dozens of free consultants on projects to keep the clients sweet. Whatever happens, my advice is keep this person involved in the core work, most often they get started on a new work package on their own and that's when it's harder to let them walk out the door.