Hi there. I am in agreement that NRE could be the cause of her lack of romantic feelings for you. For me NRE lasted much longer than 8 months with Mono. Just so you know it can be longer.
It sounds like the two of you had NRE for poly itself and then when she fell for this guy it became about him. Realize that this is all new and her love for him will become stable and less crazy in time. Right now she is experiencing romantic and lustful love for him and attachment love for you. Both are valid but can be confusing for someone in it. I started a thread on this in general discussions. The thread is about an anthroplogist that talks on this. Her name is Helen fisher. Check my stats on my profile and threads I've started and you will find it.
If I were in your position I think I would hold off on dating in order to hold fast to the reigns of your partners raging love for this guy. She is like a wild horse with it right now and just wants to run with it. Its exciting, makes her heart soar and she thinks that the open meadow she is galloping through is endless. She has been bored and now is inspired. It isn't going to last, but she will be changed. She will tire herself out and will come down to reality in time. Like fire, it consumes itself eventually. This is when you'll need to be ready for her and what change comes of this new stage. She could want to maintain a long relationship with this man and will want to negotiate that, or she could realize its not for her and she's had her fun. I suspect she will object to the latter right now
Good luck. You aren't alone on this journey as is evident if you do a whole lot of reading here. I suggest she does too as it is a great resource in many different ways. I suggest a tag search on NRE first.