I would let the topic rest until you can speak to your daughter in person and in peace.
Of course she has a right to have a say and I would not take any further steps without her.
However, I think it is extremely important to support them in not silently enduring such sexual assaults.
This guy needs to be shown that he has crossed a line (including a legal one).
And your daughter should be supported to defend herself against sexual violence.
You could go with her to a counseling center and get information.
Or she could seek support from girls in her class. I would confront the classmate who started the whole thing and gave out her number without permission. The class teacher would also be responsible in that case.
I think the advice here in the forum to simply accept everything in silence is wrong. Adults would be overwhelmed by this situation and a 15-year-old is supposed to handle everything on her own?
Of course, you cannot ignore your daughter here, but with professional advice it will definitely do her good to move from the passive victim role to an active, strong role.
