She has a live-in bf as well as a husband, and is jealous of her husband having a gf?
I'd say, go meet with her. Personally I'd want my bf/her husband there. It sounds like you have a heavy agenda though, to somehow "prove" to her that you aren't out to steal her husband (a cowgirl). Are you his first gf? Isn't she sure he is poly? She thinks he is looking to replace her even though she hasn't replaced him with her bf? Odd.
Generally it can help a jealous spouse to meet their new partner. Somehow just meeting them can make them seem less of a threat. But having the agenda of proving you aren't a a cowgirl could definitely make you feel nervous. How do you prove a negative?
I'd try and keep the conversation light, just chit chat and getting to know each other. Might make you seem less demonic.
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
me: Mags, 59, living with:
miss pixi, 37