That is what makes me suspicious of him. There is nothing as emotional and intense as walking away from a marriage, especially with kids involved.
I know how I felt when my first marriage blew up. Leaving the house that night was surreal, and I was very on edge, and bitter about his new girlfriend.
I could just imagine how JB was feeling, driving around, wondering if SB was feeling as bad as he was,then seeing her sitting on the stoop, laughing and drinking with Debra. He probably blamed DB a bit as well, because it is natural to blame the best friend for stirring things up and being a bad influence.
Did he wonder where his little girl was later on, while his wife was sitting and talking to the male neighbor? I would have been mad.
eta: Really weird that Mike Thompson identified him as the guy walking with a baby. And LE gave him a lie detector test, so they had their suspicions raised as well.