How do i deal with this 10 points best answer?

my gf wants to go swiming with her little sister tomarow but this guy who kissed her once when me and my gf were going through a rough patch is going as well i know he likes my gf and i also know he doesnt dare piss me off again. how should i react to it should i let her go because i want her to be happy but i dont want her to expose any part of her body (two peice bathing suit) around the ******* creep. when i think about it i get a roush of rage and i find miself gripping things to hard ive already cracked the screen on my phone broken my doorknob and broke an arm on one of my chairs. yes i know i have anger issues but i want her to be happy and im in a real pickle