Did she lie to me about sleeping with her friend? Girls please help?

backstory, before me and my girlfriend started dating during the talking phase she said "Pauly is my best friend he's a guy, we've never done anything but it went around that we did cause my ex was cheating on me so I set up a fake message with him making it look like it did and my ex posted the screenshots so everyone thinks stuff happened" she promises nothing ever happened between them and that her doing that was wrong she just wanted to show her ex how it felt with the fake messages. Now at first there snap messages were delete after viewing. Then she changed it to 24 hours after I brought it up. He friend that introduced us told me she'd never open his messages in front of me and one night he messaged and I told her he said that and her daughter called right then and needed the door unlocked she went out to unlock the door and viewed the message out there came back in like nothing happened and like that statement was never made put her phone down and everything. I asked too see the messages and she lied and said there not there now there gone somehow "I'm not stupid I know she deleted the thread even though she said she didn't and she recently admitted to deleting it" because he said smile beautiful to her having a bad day and she didn't want me to see that, already talked about that and she didn't understand how that was wrong 🙄 whenever I bring it up cause some people have told me no she's lying to you it definitely happened she says "it didn't happen I know what I did and didn't do but you don't trust me your believing them when I even warned you. It's my past it shouldn't matter even if I did or didn't, you believe them anyway so ill lie and say I did" I don't care if she did or didn't sleep with him honestly the part I care about is when we first started talking she promised to never lie to me or cheat on me the 2 things I can't stand ! So I just want to know if she did lie to me thats all. Anyone help?

Updates
9 mo
I will say after the last time I brought everything up she did block him on everything. At one point she came to me and said I'd even take a polygraph to prove it. Funny part is I asked later on so let me guess if I said take the lie detector I'd prolly lose you huh? And she said that or I'd feel some type of way and it wouldn't be good and now oh you'd really spend the money on that? There expensive.
Did she lie to me about sleeping with her friend? Girls please help?
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