Complex sentence: Foster became friends with A.J. Ayer although was strongly critical of Ayer's philosophy; he later wrote a book on Ayer's work that Anthony Quinton described as "the only serious monograph about his philosophy".
Split into simpler sentences: Foster became friends with A.J. Ayer although was strongly critical of Ayer's philosophy. He later wrote a book on Ayer's work that Anthony Quinton described as "the only serious monograph about his philosophy".