Talk to her about it? If she knows you're uncomfortable with it maybe she can change her plans.

Do they have a romantic history? If not, I wouldn't really worry about it. If you trust your girlfriend, then you know she'll honor that trust. You can't (or shouldn't) really control your girlfriend's friends or how she spends her time. If you have concerns, let her know. I'm sure she'll be willing to talk about it and put your mind at ease. Without constant communication, trust can't flourish.

You need to trust your girl. If something comes of that event, she wasn't really your girl. If she loves you and have proven it to you - why are you so upset? Show her you love her and tell her its ok with you that she has a friend she can spend some time with that will be fun for her.

dude fuck that hay, fuck that up the river man, i wouldnt let that happen, my fiance doesnt like it when i go away with my friends and shes visiting her parents in NC and me in aus where we live, i went away with my friends and there girls and a chick friend of mine, MY FIANCE WENT NUTZ AS A MOFO ON CRACK, we have a rule, we dont sleep at ex's house, and ex's dont stay without the other partner being there, we dont think its right, specially with history being between them, cause normally ex's still hold feelings man, personally, if i found out she did have a ex stay at her parents house while over there, ild fly over, call GBS and fuck him up and then leave her arse on the side of the street, i wont put up with cheating or fuckin lying, talk 2 her, tell her u dont like it and that it makes u uncomfortable, dont say words like, i dont think, i dont trust (specially her name, or the ex), i dont want (makes u sound like a jealous untrusting person, not 2 think a controller) and just say look babe, i feel uncomfortable with it, i dont know him, and i do trust you, but i dont trust him not 2 try and make a move on you, can you please understand im just worried and i know it might sound stupid but please can he stay somewhere else. THAT WILL GENERALLY WORK, THEN DROP CAUSE I LOVE YOU SO MUCH killa PCZ

THE FRIEND MAY JUST BE AFIEND TRUST HER BECAUSE YOUR ABILITY TO NOT TRUST CAN FORM A WEDGE AND IFTHE GUY SECRETLY WANTS HER IT COULD BRING CLOSER .SO BE EXCEPTING