My girlfriend go swimming with her friend guy, who told her that he likes her before?

my girlfriend ( F28 ) and her friend guy (M18)

well, I live in another place now, my girlfriend has a friend guy who told her that, she told me that she said no to him, they go to hangout at least once a week, she told me that he'll teach her how to swim, they go once a week, I asked her if i can talk to him and meet him, i called him once when they were together, After that i texted him nicely to know him, but he Keep me away from talking to him. that's not a problem for me, the problem is when i told my girlfriend that i feel uncomfortable about that, she defend this person instead of listening to me or my feelings. should i be mad? and what should i do?
My girlfriend go swimming with her friend guy, who told her that he likes her before?
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