tough one.
either option you have is shit.
important thing is not to get angry or threaten her. If she is texting with her ex then that is fine. Talk about it. Talk about her ex and what nice things he did. Anchor that to you now.
Be adult about it. Speak about what he tells her. Speak about the sexual fantasies he has about her (and like it)
then do it to her.
Id anchor everything i can get from this guy towards me. Make her realise that everything the dude is typing she HAS with me.
And be nice about the dude. Talking about what a nice and kind guy he is (if he is!)
be honest, unhurt and unshakably strong.
The most difficult thing is probably to get her talking about it in the first place.
Thats where she is afraid of your reaction. So give her no reason to be afraid.
You know what she does. If you reality check that she still really loves you and finds you attractive youve got a good chance of using this to become a stronger couple
if not - you have other problems anyhow.
To sum it up: I dont think the guy is your problem
I think the problem is that she feels she cannot talk with you about everything (and usually YOU are the reason for that)
The problem also is that there are fantasies towards other men (which in itsself is nothing unusual). It only gets a problem if its not only sexual and combined with emotions towards a specific man. (Its like watching porn. In itsself its no problem. If you are obsessed with an actress however and it gets personal...)
If you think I am insulting you with my post or bashing you: You do not get the point.
I am not here to insult or bash anyone. I offer up my free time to help. Take from my post what is useful to you.
If you are angry about my post or myself, then please stop and think how that happened. Usually that is the way the brain responds if a critical belief system is challenged (its called cognitive dissonance). If you have trouble with it please answer in the thread. I will come back to you.