Yikes. I don't blame you. No, moms don't text kids, and certainly not to say "no more friendship" - so the other girl's mom is completely wrong and engaging in too much tween drama. Kids need to learn not to have relationships by text, first of all. They need to talk. And the other girl is jealous and not ready to have multiple friendships - it's sounding very kindergarten-y.
I think I might have called the other mom rather than perpetuate the texting thing, but if she's so aggressive, maybe you did the right thing. I don't know her and you do.
If they have issues in school, let the teachers handle it. Stay out of it. Don't take any steps to move classes around or any of that. We all need to learn to go to class or to work with people we don't adore. Teach your daughter to be pleasant but detached.
For now, I'd consider blocking those numbers from your daughter's phone. The mother doesn't want them in touch anyway, so you're not actually doing anything defiant in her eyes! Maybe it's not necessary - just encourage her to have other friends over and to move on.
Encourage your daughter to have face-to-face relationships and to recognize that texts have no context and no voice inflection and no eye contact. They are limiting and they don't help kids foster real relationships. I know you're shoveling sand against the tide if you say that, but studies are showing kids are really hampered in work and personal relationships because of the reliance on texting. So think about your position on this before she enters the teen drama years.
Don't draw a line on "not welcome in my house" - it's too final and it may serve to drive your daughter to this girl. Let there be a cooling-off period and say "you need need to let her and her mom calm down". Declare a moratorium on contact for maybe a month so everyone simmers down and both girls can see if they actually miss each other. Your daughter may see that this girl is needy and insecure (hence not wanting other friends) so tell her to be pleasant but not overly solicitous while everyone heals and looks around for other options. Then it can be revisited. Telling a tween or a teen "no" can push them in the very direction you don't want them to go in.