Open communication is the MOST important thing. In our experience being the husband and wife, having a person live with us for a year, talking about it was the most important. As long as you don't hate her and still enjoy her company, there is no reason for her to feel bad. Just because you are more "into" him.
Sounds like you have great potential for a poly relationship. We would love to find someone like that again.
Thanks for listening and hope this helps.