I was thinking about how the others character quests affected Jay and his decision in his quest. For the whole main game and some of the character quests, Jay spends a lot of time with the party and bonds with them. They are the first group of people Jay has actually bonded with, he doesn't bond ith clients and the Oresoren are well otters.
They are the first actual human companionship he has actually ever had and experienced. This makes the following events have a much stronger and negative impact.
So because of this when the quests actually start and when the others start showing doubt and caving in, while he doesn't show it or tries his hardest to, Jay becomes upset. The more and more the quests went on, the more and more it became harder for him to hide his feelings which show in his remarks and general behavior.
By the time it got to his quest I'm sure he had come to terms somewhat with everyone not being so strong willed as he thought. But Jay being Jay once proved wrong becomes distrusting and with him still somewhat shaken up the events of his character quest easily get the best of him.
So when Solon returns into Jay's life he's thrown for a huge loop. Jay's little enclosed world had been shaken by the revelation that everyone in the group was hiding things from him. While no, he had done nothing to warrant getting such information, he still went into these relationships with the expectation that they would communicate to him without knowing much better.
Solon's appearance shakes what little hold Jay had left of his little enclosed world. It makes him question everything, which leads to the doubt which brings Schwartz to him. What if his so called friends don't really want him there after all? What if the don't want Jay the person, but only Jay the information dealer and only tolerate him being there because of that?
This clearly helps him make his decision when he's backed into a corner by Solon. Why keep friends who probably don't want you there, when you can have the family that does want you instead? Once he helps kidnap Shirley though, he probably realized that he just did the same things the others did. He hid things from them and didn't express anything to them. He feels shamed and hypocritical, how does one show their face after they do that? Let alone the looks on his friends faces, I'm sure he also heard some of the conversation afterwords like Senel's rage or being called a traitor.
Though he's made his choice and sticks to the deal with Solon. It takes Shirley's verbal beating to finally snap Jay out of his trance after the Oresoren appear.