This mother is out of line and out of bounds. She is thriving on the drama and the attention. You've actually fueled that by speaking to her at all, let alone by telling her she's getting to you and that your son is not allowed near her daughter. Now she will say her daughter is even more the victim of your son.
A minor point - this girl texted your son? I'm not sure why your 10 year old has a cell phone but I'm hoping it is ONLY so he can reach you and someone else in case of emergency. Block her number NOW. Tell him if he engages with her or any other child on matters like this, he loses his cell phone and a few other privileges.
I have NO idea why you would think it was a good idea to intervene in a disciplinary matter that the school was handling. The school has a protocol, and your involvement should be with the school only, not with the other mother. Do not talk to her, and block HER number from YOUR cell phone. If she approaches you on anything, tell her succinctly to make an appointment with the principal.
I would set up a conference now with the principal and perhaps the classroom teacher, and YOU. If your child's father is in the picture, then include him - no one else unless there is also a school psychologist or a professional of that nature. Not the other mom. The teacher can take whatever steps are necessary to make sure that this girl and your son are not seated together, not assigned to a team project for anything, not lining up together.
But as a former teacher, I can tell you that learning stops when teachers are expected to supervised every interaction and report back to parents. That's why I disagree with the suggestion below that you should be in the classroom! No you shouldn't! That would just put you in the same category as the interfering mother. You DO have the right to request that a neutral party be in the classroom, someone with expertise and someone not connected to either this girl or your son. The school psychologist, or a guidance department member, or even a paraprofessional aide would be better suggestions. Let them observe what's going on but appear to just be classroom visitors. If you are informed that this will happen, UNDER NO CIRCUMSTANCES should you inform your son of this! This is not the sort of issue a 10 year old should be handling. His involvement with the girl should be to have one or two set phrases to use when she interacts with him. These phrases should be supplied by the teacher or principal - he should not answer her, he should only say the key phrases. Those might be something like, "Excuse me, I need to speak to the teacher." (Not "I'm telling" or "I'm not allowed to talk to you.")
I agree that you should write to the principal to request this meeting and that it should be factual with dates and only verified info. (If it's just what your child told you, it puts you in the same category as what this wild mother is doing.) I agree that it needs to be re-read by someone else before you send it, for objectivity, spelling, punctuation, etc. I understand that you are venting on mamapedia, and that's what we're here for, but a lot of your post was written in anger/frustration and I agree that it is rambling a bit and difficult to understand. It would be easier for the administration because they are familiar with the incidents and personalities involved. But still, it should be proofread with proper spelling and grammar, and a logical sequence. Be sure to say what you are asking for (such as a meeting, and certainly some guidance). You should say also that you respect their expertise as well as confidentiality for students (that lets them know that you understand they are not going to share with you anything about this mother and child or what the underlying problems are or what has already been said to them. Don't fawn over them, because that doesn't ring true, but don't blast them either. For all the contact you've had with this mother, they've probably had 10 times as much. It's okay to admit that you were in touch with the mother but that you now think that wasn't a good idea, and to ask for their advice on what you should do/say to your child.
Leave out the fact that your child is a straight A student. That's very nice for you and for him, but it is not relevant to his behavior or his rights in the class. A good student can be a bully, and a poor student is entitled to just as much protection from a bully.
The calmer you can be in your dealings with the school and with your son, the more progress you will make and the more your son will see the best course of action to follow in a dispute. You go to the authorities, you don't expect 10 year olds to deal with parental issues, and parents are their child's best advocate IF they follow the protocols.