I've seen a few qualified PPLs who, for one reason or another, just didn't come up to my standards. A common reason may be that they originally qualified but then never did enough flying to properly consolidate that, another is that they reached the standard required to pass a PPL skill test or checkride, regarded that as the peak of their achievements and then happily slid downwards. Whatever the reason, you're quite right to hold your ground, and be open as to why you're doing so. In my experience however, the lower the standards of the pilot you're training, the more resistant they usually are to being required to up their game.
I would advise however complete openness, say what you think's needed, and why. Be open when somebody isn't improving, and what you think needs to be done about it. And don't be shy of simply saying "you need to go spend quality time with a proper instructor, who is qualified to work on your basic flying skills". I know you're an experienced and capable pilot, but that's not the same as having been trained to actively address missing basic flying skills.
G