Quoting MyMonkeysNAMEisFINGERS:" Can you go spy on him at work? Or get someone else that works with him to spy for you? I don't care of it's sleazy, I would do it if it means saving my marriage, or lack thereof."
Spying on him won't solve anything. She already knows he's having an inappropriate relationship with this woman based on the fact that he's sending her so many texts and getting extremely defensive about the nature of their supposed "friendship." Besides, it will only add to the paranoia and more or less make things worse in general.
In other words it won't accomplish anything since she knows he's already doing something he shouldn't be doing.
OP, what I would do is sit down with him one more time and lay everything out on the table. Collect all of the evidence you have by printing out your phone bills with copies of the texts and place them in front of him. Ask him how he would feel if you were talking to another man that much. If he says he wouldn't mind he's full of crap and let him know just that.
After that tell him to stop treating you like an idiot (which is exactly what he's doing by pretending nothing is going on). It doesn't take a genius to figure out that 500+ texts a month to someone of the opposite sex when you're married is completely inappropriate.
Tell him it's not the fact that she's woman that bothers you but it's the fact that he's so involved with her that eats you up inside. Not only this but let him know how much it hurts to be so very disrespected by him. By ignoring your requests to stop talking to her he's disrespecting you on a whole other level and that's not okay. It's not like you're asking much of him to begin with anyway (and remind him of that!).
After you tell him these things give him an ultimatum. Either he can stop talking to her and start seeing a therapist with you or he can expect consequences. What those consequences are is up to you but I would personally threaten a separation (not
a divorce, just a separation). He needs to know he can't continue on with this. I would also remind him that relationships in the workplace are considered inappropriate and they could get fired for it if they don't watch out. It doesn't take much in a lot of places and if their boss were to find out they're texting one another constantly s/he would probably be none to pleased.