He might well fall in love with her. Would that be ok with you? It by no means necessarily means that he would fall out of love with you, as long as he is paying close attention to his feelings and making special time for you even as his relationship with her develops.
Maybe it would be better if you guys were more clear that group time, when everyone is together, should be less lovey-dovey (which means, for the sake of her feelings, that you and he also have to keep your hands off of each other when you are all together) and that lovey cuddles should be reserved for private time for now?
Great resources here, in the Jealousy and Insecurity section: http://www.morethantwo.com/
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.