my girlfriend slept with this guy during a period when we stopped dating, he had attempted to kiss her when we were dating and she told him she has a boyfriend
She asked me tonight if she can go to his party she told me she hadnt talked to him for a while and wanted to be friends with him.

We had an argument today earlier this morning and after we'd made up she asked me, i'm guessing that she initiated chat with him as she did 2 other times we broke up.

I told her i feel completely uncomfortable, i trust her enough but i dont trust him, obviously i want her to not go to this party but im not gonna tell her what to do.

why not she is your girlfriend. of course, you are not an abusive person. but a committed relationship means just that. you have the right to demand certain behaviors (or lack thereof). as much as your girlfriend would be in her full right to demand you do not hang out with a girl you slept with.
this is not a matter of trust, it is a matter of respect. it would be a matter of trust if she casually walked into this dude on the street and just said hi and chitchatted for 5 minutes. then, yes, it would be a matter of trusting she is saying the truth. but this is plain and simple disrespect. and you should stand up to it, going next included.

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nice guys don't get laid
"It's disrespectful not to bang them when they sleepover." (Hellhound)

NO

Depends!

You should trust your girlfriend to go and assume she is going to be respectful towards you there, not flirt etc. You can't and should not try to control her man, YOU ARE THE PRICE! She must know that if she slips up YOUR GONE!

Telling her not to go because you don't trust the guy makes yourself look insecure.... This guy is nobody compared to you! By telling her your afraid of him seducing your girl, your pretty much saying he is a better man the me and iam afraid, this putts him in a better spot!

Now on the other hand! You could also next her ass when she goes because it is very disrespectful, slept with the guy, he hit on her and yet she wanna go to his party......

Now just saying: "If you go its over"... quite childish right? say something like: "its your decision but i rather not have you go, i find it very disrespectful towards me."

Now if she goes anyway she broke your respect boundary and you should NEXT her ass! If you don't she will not have any respect for you anymore! Man up

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Do not make external things like girls define your happiness or you will live a harsh life. Keep doing what you love and keep improving as a man.

??

Trust your gut feeling.

You feel comfortable with this situation, aside from pua things you've read.

You trust her 100%?

Let her go.

You feel akward and not correct.

don't let her go.

Two things that i notice here.

1. She says she wants to be friends, but he's listed in her head as a lover (They had sex)
2. She tells you that she is going, and even asks permission. She wouldn't if she had bad intentions.

this is really just a giant grey area, this is a personal boundary issue, I mean it is one thing to be super jealous and insecure trying to shut down your girlfriends fun by never wanting her to be going out and worrying yourself to death, but it is another thing all together if she has a history with a guy and you are certain the guy will try something on her and she wants to go hang out where he is, in a place where he can get her alone, where drinking will also take place, this is like locking a fat guy in a room with a giant cake after he hasn't eaten for 2 days and trusting him to not eat the cake just because he says he won't

you have already expressed to her that this makes you feel uncomfortable, and she is choosing to do it anyways, there is no point in trying to control her or tell her what to do, she is going to do what she wants to do anyways, and if she wants to see this guy, guaranteed she will either with or with out your knowledge, rather it is just a clear sign of dis-respect towards you and a lack of empathy to how this makes you feel

if that was my girlfriend, I would probably just express my dis-comfort with this idea (as you already did) and dump her if she goes to the party unless I was considering the option of sleeping with other girls and having an open relationship

what she is doing is the almost the same thing as, hey baby my ex-gf called me tonight and wants me to hang out and get drunk at her house, do you think your girlfriend would be ok with you hanging out with an ex alone and getting drunk with her? have a little respect for yourself man, one girl no matter how much you like her is not worth your own sense of respect and self esteem, it will only damage you when it most likely has nothing to do with you, but rather everything to do with her and how mature she is, if she is doing this kind of shit it is a good sign she is not mature enough for a relationship and even if you like her there is a good chance this thing is bound to not work out

Aren't you invited too?

Asking for your permission is the weirdest thing, girls do that when they are up to no good, usually just so she can justify her actions by saying " well, my BF let me go".

I don't really recommend the usual PUA mumbo Jumbo about "No, you are Alpha, do not care", i used to do that and ended up with a cool gf, but she used to fuck her friend when i am not around, beause i am too alpha to ask her why she's hanging out with him all the time; get my point?

If your guts tell you she's up to no good, you tell her that you don't want her to go, she doesn't, you reward her, if she does go even tho you said no, you mentally next the bitch.

Clearly something is wrong.

Tell her:

"you are free girl and can do what you want. I just feel it isn't very respectful. But hey... go ahead."

It signals you are not ok with it, but still it's her choice.

If she still will go, then make your own conclusions out of it.

Perfect response! +1

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Do not make external things like girls define your happiness or you will live a harsh life. Keep doing what you love and keep improving as a man.