It all depends on what your WS uses as a definition of "love".
Did he have some type of feelings for her? Sure he did. He cared for her on some level. They were friends, they talked a lot. He enjoyed spending time with her. He obviously enjoyed having sex with her. They fed each other's need for attention.
He probably compartmentalized their sexual relationship apart from his "friend" relationship with her. And he probably thought is was really cool and hip that he had a FWB relationship with her.
Would he want to divorce you and marry her? Ask him. Chances are no, he liked his life with you.
She was just the "strange on the side".