One thing that I don't think Matt understands in all of this is you can't just tell someone to stop loving someone else. Ry and Si's feelings for each other are real. They're not just going to magically go away because Matt doesn't want to share his wife any more.
Did anyone ever stop to think that maybe she needed a break from spending time with her husband; needed a bit of girl time away from him and his family? To suddenly go from living near her family to living thousands of miles away near his family is a huge change. I bet Matt would be happy if she didn't make any friends and spent her while life focused solely on him and the children, but that wouldn't be fair to Ry. She's worlds away from home, ofcourse she reached out to a friendly face. Yeah, she fell back into old habits, but I really don't believe that it's ever possible to put the poly genie back in the bottle. Once you've known multiple love, you can never go back to being mono, thinking mono, etc and for Ry, who has never been mono before, it has to be doubly hard to try to adjust. It's like asking a mono person to be poly. I really hope that you find a balance that works for everyone.