Give her some time to come to terms with the idea. Awhile back, I flirted with a woman. I told her that I had a gilrfriend and explained polyamory. (This was like 15 years ago.)
She kind of wanted to play it by ear. Basically, I think she thought she would have a fling with me and then walk away with an experience. She kind of saw it as friends with benefits or "being the other woman" while I was cheating on my girlfriend.
However, as the relationship developed, she realized that it was more about love than just friendly sex. That took some time so she could get her head around that concept. She surprised herself by how she felt about everything.