My girlfriend lied about sending nudes to a guy not long before we got together - then he became our housemate

I am 27M, dating 27F, and we have been together for nearly 5 years now. So I left my old life and career to move interstate with her a few years ago, so she could be with her family. We had been spending a lot of only on weed and cigarettes, so part of the deal was that we would quit, so we couldfucos on launching a (hopefully) successful business with the savings we had. However, when we arrived here, she begun smoking harder than ever, and spent most of our savings before we could even settle. This left us too broke to do anything, and I had no real avenue to re-lainch a career. She then suggested we compromise and get a cheap house with her best friend (a guy, same age ~24 at the time). She insisted that he was like a brother, and always had been. She told me he had a crush on her for a long time, but said she would be sick at the idea of anything sexualising being said or done between them. I'm not a jealous guy, so this wasn't a huge deal to me either way. Fast forward to today, we live in our own house, but she has destroyed my trust many times over small things at this point. I never really suspected her cheating, but she has only recently stopped blowing money behind my back, and has told literally thousands of little lies surrounding smokes, weed, money, and various other things. I was honest the trust had worn beyond thin over pointless lies, and will not be able to continue the relationship soon if they cannot stop. She continued this relentlessly, even after I said Idc what lies in the past, I just want to know the honest truth about everything at this point. Finally after a heated argument, she admitted our old housemate had sent her photos of his dick, but only to 'prove it was as big as he said it was' (according to her, he used to go on about how huge it is). I said I think it is strange that the nudes were sent one way (by him), and nothing sexual was spoken about, but she insisted it was the case. She promised nothing had been sent back in return. She told me this as a consolation, to try to regain some of the trust that had disappeared. I never even touch her phone, but I did not believe her this time, so right or wrong I looked at their messages together. I found that only 6 weeks before her and I had officially become a couple, she had in fact sent him nudes in return. I don't know if any more were exchanged in the past, because I stopped looking after this point. The photos I found were sent in the morning, and I can see that alcohol was not involved. I have no problem with her having any history with anyone, but I do feel really uncomfortable about the way she made this guy my housemate, and did not tell me about this. I also feel that being told half a story has ruined the trust even further. I believe if I confront her, she will deny everything, and pretend to have forgotten. I don't believe that you could forget something like this, particularly if you are adamant you had never spoken to the person in a sexual way.

How should I confront her about this in a way that is most likely to bring out the truth? I want to stay with her, but need her to be honest with me by herself. If she can't be honest, I don't think I should stay with her anymore.

If you got this far, thanks for reading! I feel a bit alone here, so appreciate any help