hi there and welcome. My question is, do you know the boyfriend? Is he aware of you? Do you and he get along okay? What does he think or all this?
Once those questions are out of the way and you have the green light to start something with her then I would ask yourself, what do I want from a partner? Do I want to spend every waking moment with them or am I okay going about my life and seeing her every now and then... the reality is with poly that time is a huge issue. It will be cut in half for her. Well actually three as part of that time she needs to spend on herself. If you are an independent person that enjoys having your own life and interests then you should be good. If this is something you could handle and be willing to embrace, then you could be good to go...
I would also wonder if you are interested in finding someone else who is mono and are keeping her around until you find someone suitable to you, or are you interested in poly and would look for other partners. Some poly people get really hurt when a partner ditches them after their mono partner finds someone else that is mono... if you care about her, it could be something to consider before getting involved.
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