Is it acceptable for my boyfriend of 5 years to talk to his ex's friends.....?

he says he has known her longer and he is not interested in in his ex. Its just he lets on and speaks to this friend and it obviously makes me a little parnoid if he asks about his ex etc. He was only with her a couple of years and he was a lot younger and we have beentogether over 5 years and live together.
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