It may not necessarily be a problem, but it's a scientific fact that the transmission of energy is most efficient, when all finger joints are bent (see Charles Duncan's discussion of this topic in "The Art of Classical Guitar Playing"). Hence, collapsing LH finger joints require more energy, which is especially detrimental for slur technique.
Absolutely, my only point is that it may not be fixable.
Get him to hold his hands palms towards him and then bend the first joint of the fingers down. If he's like me, the fingers will form a semi-circle rather than a straight row, with the fourth finger on the left having a particularly pronounced lean. In my case its worse on the left hand than the right, implying that years of playing fretted instruments have actually made the problem worse
- so much for exercises!
I wonder what an orthopaedic professional would say?