p.s did her ex break up with her? or did she break up with him?

I agree with Pumpington here, and it took me a while to get to that state of mind.
Ive flirted with girls that have had boyfriends... and most of the time they don´t have the intention to cheat, they just like the attention and validation from the opposite sex, even if they say they don't need it, they want it, it doesn´t matter how many times she denies it, girls love that shit, but it doesn´t mean she´s a cheater. There´s a particular guy that has been hovering around my girl for about a year or two, and they´ve both had said they wanna see each other, and hell, obviously the guy wants to, but she´s never met up with him, and she told me at the beginning ¨Look if I wanted to see him, I would be with him and not with you¨ ... I know its hard to accept, because maybe you wouldn´t do it, but as long as she´s not secretly meeting the guys, or actually cheating on you, she isn´t doing anything wrong.
Just trust her.

There is a big difference here! She is not talking to just any guy, she is talking to her ex, and the flirting goes beyond innocent flirting, she is telling this guys she loves him.

Like I said before you should never put your self in a situation were you could cheat. The thing that many neglect to understand is that no one is making anyone eles be in a relationship, its a choice, and that choice comes with certain responsibilities. One of which is not hitting on ex boy friends lol

might be true. but still, there should be a line beyond which the flirting is not innocent, and even if it is not outright cheating, it qualifies as disrespect.
I can see some giggling at jokes, playing with one's hair, ..., that kind of stuff is mostly innocent.. can feel unpleasant if you know what is going on, but no reasonable person would call out their girlfriend on "hey, you're scratching your elbow too much"

but if I found out my girlfriend were texting to her ex "I love you", that would be very disrespectful.. even if there is no actual meet-up.. it would sound like "yeah, my BF is a joke, but you..you're the real deal"
it might not be cheating, but I would call out her on it..

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

Well, it can be true, texting ¨I love you¨to an ex boyfriend, is probably a red sign.
Sometimes we think too much of what we would do in such cases, but we don´t really think about how the girls think.
I´ve had girls that have said they love me even when having a boyfriend, yet, they never agreed to meet up, or said I was only a fantasy.
Girls talk a lot, but they don´t really mean anything.
What could be a good way of adressing this to her? Don´t do it! .. If she´s not meeting him, then things are not bad! .. She´s not seeing him behind your back. Girls sometimes just do that to follow along, sooner or later the ex is gonna get tired of it, and is going to stop contact with her, or she will get bored of him.
I know its something hard to accept, but keep in mind if she wanted to cheat she´d do it