Should i leave my boyfriend, of a year and a half?

My guy is good friends with his teenage ex girlfriend, her husband, has no idea, they hang out. Before he was my boyfriend, they would flirt and I'm sure they still do. Their friendship has me worried. He says it would never go any further than just friends. He recently moved back to his hometown. I'm supposed to move with him in a month or 2. He knows exactly how I feel about her. I've never met her, but i know i can't stand them being friends. Our relationship is the best we've ever had. I Definitely need some advice, am I over reacting, or should I just walk away?