I was a consultant for a while and trained more than a few FNGs. I would always advise them that the choice was theirs, but that the chances of a positive result were slim to none if they took their concerns to the customer. Sure enough, I saw several otherwise excellent consultants get shown the door because of this exact scenario.
Part of your responsibility as a consultant is to "work magic". If you run into roadblocks, you find ways around them and that includes the occasional professional vegetable. What typically isn't in your domain is giving advice on personnel, unless you were specifically hired to do exactly that. In the end, almost nobody wants to be told that one of their chosen workers is sub-par. It's negative, it's dangerous, and it's usually pointless.
Just work around them, document everything, and communicate that sort of stuff with your own manager behind closed doors. You should also be sure to have customer "witnesses" in your emails and meetings. Team distribution lists and direct managers are excellent for that.