He told me he had always been a cheater but he wants to be honest now. I want to trust him but he def is hard to trust, idk if someone can just stop cheating and lying when they have been doing it all their lives
Yes. I can see where it is hard to trust him because that part in bold could just be the "new song." Still manipulating, just in a different way. Like "honest now" and not really
honest. You seem to recognize that. Additionally past experiences tell you he says one thing and does another. So why would he change?
she knows he lies and just let's him get away with it
Her well being is HER responsibility. You don't have to be involved in that.
You are responsible for YOU. Could focus on that instead.
I call him out and we argue.
When you call him out, if it just ends going in argument circles with no actual change in behavior? Why bother?
Instead you could form a personal boundary for YOU to follow. Like "X lies and I'm out of there." Then you just count them.
He does it again and hits X number? No arguments, fights or "circle" conversations. You just say you are done and leave. Because that is all you get here and you have all the X times to show it. Why expect different?
She told me she let's him get away with it because she knows he still cares about her.
Again, her well being is not your concern. It is her concern and her responsibility.
I am talking about things like what he does with his time and such. So ik not sure I can ever trust him, I should leave like yall are saying, he has a really large penis that makes me not want to.
Are you saying "I want access to his big dick. So I am willing to pay the price tag of (putting up with his poor lying/cheating behaviors.) Even if after paying it, I wind up feeling upset/used/demeaned" here?

If so? Pay the price tag. If not? Stop paying. I can imagine it might be hard to FEEL right now. But the actions seem straightforward.
In you shoes? I'd stop paying. There are are other big penises in the world attached to non-lying men. Could find and date these more honest, respectful people instead. Could also buy a big one in the meanwhile -- even amazon sells toys. Why sell your present quality of life and peace of mind short over big penis?
Life is not a dress rehearsal. You only get the one. Seems like a waste to spend your time and energy on a relationship you KNOW is not healthy just for big dick. It's going to feel hard to feel proud of that behavior choice.
Could masturbate with all the big toys you want while you heal and then move on to date healthier people. Last I checked, toys won't lie to you, cheat on you, argue with you, fight with you, etc.
Choosing to stay in a thing you know is not healthy when you know you ought to leave? That is not a good sounding choice to me.
Galagirl