After finding out that Gwen was sleeping with another man, Fidelis was heartbroken, but there was nothing he could do about it because he wanted to protect his family's reputation. All these years, the woman prided herself on her infidelity while still trying to take the throne from his only son. With the years that had passed, Fidelis wanted to let Gwen change by herself, but the woman never did, and she was now shamelessly sleeping with another man in his palace.
The first time they arrived at Orion, Fidelis had gone out at night for a walk when he found his wife in the arms of another man shamelessly. It broke his heart to watch that, but he didn't know how to punish her for it.
Now hearing Oriana say it boldly, he felt free and wanted to really save himself finally. If Oriana was a chance from heaven to liberate him from an infidel wife, then he wasn't going to let the opportunity go to waste.