Hi,
I've posted a similar problem in viva violin couple of months ago.
I too have an adult student learning violin under me, she is borned without a left thumb and her right thumb is just hanging by a bit of flesh(meaning she can't grip the bow the same way as the others do). I'm worried about her shifting as she gets to grade 3 this year, but she is very ambitious to become a music teacher!
And now she is learning piano too! She wanted to do piano exams (violin exam is still ok for her as you don't need your left thumb to play the notes), but i can't imagine playing scales with 2 fingers off! I just adviced her to just learn with alternate fingerings (eg play a tonic triad with 5-4-2 in left hand) and to play whatever pieces she can for pleasure.
It is their interest in music that makes them to continue learning. In my opinion, it is good to let them do to whatever stage they can, and not to discourage them of their interest, and thus they may feel like we are looking down on them. It is not really impossible to play with the little finger of the left hand not used, since my student sounded worse than yours