Complex sentence: During the visit, they learn that David's violins were quite valuable, his own an Amati and his father's, which he'd given to a blind friend to learn on, a Stradivarius.
Split into simpler sentences: During the visit, they learn that David's violins are quite valuable. His own is an Amati and his father's, which he had loaned to a blind friend, a Stradivarius.