I see lots of "she should have, she could have, she shouldn't have's" about your bf's gf's feelings.
Telling your bf to tell his gf "don't feel jealous..." well, it just doesnt work that way. Jealousy is an emotion that needs to be felt, and worked through, and finally let go of when one is secure. You can't just *poof* it away.
However, even if she has always been interested in polyamory, thinking about it and actually doing it successfully are 2 different things. I shared my story with you to show you how that went down for me. I was completely overcome with surprise at the intensity of the NRE my husband had for his gf. They didn't keep it in check, and I wasn't prepared to deal with it, despite having read The Ethical Slut. I ended up feeling more like his mom, like a mother sending her teen son out on dates, and like the babysitter for our kids, doing the drudge work while he was out having fun.
It can really take a while to learn the ins and outs of polyamory. Your heart does get dinged and even broken sometimes.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
"Master," (mono, 34), miss pixi's Dom for 2 years