Yes you can be angry. But then you need to admit to reading her texts, which would make you also at fault. You probably need to talk with her about this issue to find out why she lied and why you doubted her. Or something.

As a kind of side point; there's a lot of grey area with 'lying' because simply not mentioning (a perhaps significant) fact both can and cannot be classed as lying. I'm saying this because maybe she just didn't think it was important, or that it would be 'best' to just not mention it. But idk.

tl;dr - You are justified to be angry.

If you found proof that she'd been lying to you through an invasion of her privacy, it doesn't mean that you don't have cause to be angry. Two wrongs don't cancel each other out. It means that you each have cause to be angry at the other.

Whether it's morally justifiable to breach someone's trust in order to expose another breach of trust is a question for /phi/. But as far as your relationship goes, I think it's definitely a good idea to confront her about it. Don't lie - be upfront about how you found out. There's going to be some shit to deal with there, but you can deal with that separately from the issue of her lying to you.

Just dump her ass without giving her a reason.

Hanging out with exes is very rarely appropriate. Unless she had a child with this guy, there's no reason for her to be having an active relationship. Facebook friends or whatever is one thing, but spending late nights together is not okay. I hope you got rid of her, OP. She is not invested in you and doesn't respect you.

>>2459
this, she's obviously fucking him

>>2485
>>2491

Both good points, but it isn't 100% for sure that she is actually fucking him. Don't jump the gun and assume that, but certainly don't rule it out.