Do you mean this:
https://rheuma.netfirms.com/books/lectures/lectur9.jpgHow old is your student? My 9-10 year old students have collapsing fingers, and I think in their case it is just that their finger muscles have not developed enough yet. It takes awhile to learn to control the muscles enough to prevent the fingers from collapsing.
Or, perhaps your student is like me and has Ehlers-Danlos Syndrome. It is a connective tissue disorder with several main subtypes; I have Type III, hypermobility type. The primary symptom is hypermobile joints and frequent dislocations. I dislocate something daily...everything that can be dislocated and even some joints that I
didn't think could be dislocated!

In my case, I have played piano for close to eleven years and am a music major at college. My finger muscles have developed enough that were it not for the EDS, my fingers shouldn't be collapsing. But as it is, it is completely out of my control and something I just have to adapt to.