when I started learning to fly heli's like most people I found hovering "difficult" the more pressure the student is put under, the longer it takes for him / her to get it, then for no apparent reason, you can do it, and you wonder what all the fuss was about, I don't class myself as a SFB, and I do recall 1 instructor, getting, um " annoyed " with me, it didn't help at all
in fact a change of instructor was all that was required.
I'd suggest, being honest with your students, and if you feel he/ she isn't going to make it, a check flight with the CFI should be arranged, but, don't tell the student, untill he / she arrives for the lesson