I agree with what everyone has said, but I also think having her travel to see him is a lot of added stress. Its not something I personally would consider as a "first" date. It leaves your brain to go wild with jealousy and what if scenarios.
Secondly, where does HE stand? Is he interested in sharing her? This could help with jealousy. Understanding his motives for enterring into a poly relationship with her. It might also help with your jealousy.
She will go through some suffering. Check out my blog in the lifestyles section. I was brutalized by emotional upheaval at the idea of being poly. I know it existed out there, but didn' tknow I could love more than one. That had a real affect on me