Charlie and Jillian both have histories of mercenary work. Jillian may have fought against Archons on occasion, and may have sometimes fought alongside them, if her clients ever hired both her and Charlie. Given their sides' extremely different approaches to mercenary work (Charlie is very well known and works hard to further his reputation; Faq had a history of being veiled and took steps to have no local reputation at all), it is unclear how often they competed for the same contracts or were hired by opposing clients.
Jillian never trusted Charlie, but did talk to him often enough to make taking his calls habitual.
As a former mercenary, Jillian probably understands Charlie's mercenary mindset better than any other Royal, which may be why she doesn't assume that Charlie and Stanley would become allies based on a common anti-Royal ideology. As a former Barbarian, she is also much more willing to take calculated risks than most other Royals, and so is willing to risk dealing with Charlie. She's confident enough to believe she has a good chance of benefiting from the association.
As long as Stanley and all of the other members of the Royal Crown Coalition II refuse to deal with him, Charlie needs Jillian badly. He's likely to be careful not to antagonize her, because he probably knows how easily provoked she was as a Barbarian. He knows she leads the newest and probably weakest member of the RCC II. If he wants her to be able to influence the decisions of the RCC II, he needs to help her build up the power and influence of Faq as quickly as possible. He probably knows that Faq obtained much of its income from doing mercenary work, and may be concerned that it could eventually become a competitor once again, but given the magnitude of the crisis he faces, that possibility is likely to be of comparatively small importance.