quite possibly - i am not sure of the answer. But have you heard of a tendon stretching too far? Perhaps it is damage to the connective tissues in the muscle that is the problem?

How does a tendon lengthen so far as to become a problem like this without signs of damage?

If it is truly lengthened, then you should see a difference in the length-tension relationship. Therefore, his power generation should be better in increase shoulder extension - say 10 degrees. This should take up the extra slack and should be about the same as the contralateral side with the same weight - well worth a try!