I don't "text", but can't he just block her?
Make him change his phone number. That sends the message pretty loud and clear to the OW. If she gets the new cell number and continues, call the phone company and have them block her number from calling or texting him. Also have him change his email address and delete the other one.
I have to agree I was very much like you. I didn't really blame the OW. My husband was/is a grown man and he made the choice to cheat. i could care less about her. As long as she stayed out of our lives like my husband told her too. I never really had to much problem from DH's ow thank god. I know the husband of the OW contacted me to blow their affair out of the water. Which dh had to admit it was a sexual affair and not an emotional affair like he wanted me to believe.
But that was all the contact I received.
I don't know what I would do in your position. I will say on my last dday (the day her husband contacted me)I did contact the OW let her know that my husband was hers she could have him. I wasn't going to "fight" for him. I thought they were both disgusting people at the time and they deserved eachother.
I really wanted her to know that if he wasn't with her it wasn't because I was stopping him. It does not mean that OW should have no accountability for helping ruining another family. But I am not one of those women that felt owed anything from her. Her misery alone was comfort enough. Because I knew that by engaging in an affair you were obviously at the lowest part of your life as well as the Married spouse. Miserable people like to attract eachother, right?.
Maybe try blocking her ASAP; If that doesn't work and IF she texts him again which would be a 3rd time, I would have him text her and say something like....
"leave me alone do not contact me, trying to fix and repair myself and marriage to be a better husband and father to my family"
Hugs, t rebuilt