Don King first met Jillian at the moment when he is reassessing his views on the importance of Royalty. He realized that Stanley posed both an ideological threat (because of his claim to a new Titanic mandate) and a strategic threat (because Gobwin Knob had just used an unknown weapon to wipe out an entire army). Jillian probably interested him because she is a young Royal with a great deal of combat experience who had good reason to hate Stanley and who had demonstrated some capability to predict Stanley's actions. He decided to help her rebuild Faq.
But then, he learned at some point that Ansom, whom he knew Jillian loved, was not, as previously thought, croaked. Instead, Ansom was leading Stanley's forces as his Chief Warlord. This may have led Don to question how Jillian would react to Ansom's apparent decision to support Stanley. Could Jillian be trusted, or would she join forces with her lover Ansom? It may have been concern about which side Jillian would fight on that caused him to demand that Jillian pop a Royal Heir, because producing a Heir would dramatically slow down Faq's ability to pop other units. When Don learned the truth about Ansom from Queen Bea's final message, he probably realized that his assumptions about how Jillian would react to Ansom were wrong. If he believes she will regard the new Ansom as an abomination, then Don is likely to trust Jillian more than he did before he learned the truth about Ansom.