My girlfriend might hang out with one of her guyfriends at the pool tmrw..how would you feel about this?

My girlfriend might hang out with one of her guy friends tmrw at the pool...her and me have a really good relationship but I'm kind of iffy about her hanging with this guy friend because he has said some sexual things to her in the past like we can have sex while we me and her were dating...and she told me that he said it didn't agree with him and said she has a boyfriend for that...her guy friend has a girlfriend...but anyways they might be going to the pool tmrw together...she says that's like her brother to her...what should I do? And how should I feel about this? How would you feel about?