c0ncerenedmum hi, how's it going, I am presuming as it is holidays it is really hard to tackle this.
This is basically sexual harassment or hate speech or both. If these comments were racist in nature he school would all over this, but because it is misogynistic then they may not be so quick to act.
I really do not know what you should do but in your shoes I would probably get her to message the boys, or you message them on her behalf, and explain that this is harassment/bullying.
I'd say I have screen shots and will take these to the school on the first day of term.
I might give them a chance to apologize by text and not repeat the behaviors with the warning any repeat offence in person or by text/on line and you will take screen shots to school.
I might also take a look at advice from charities concerned with bullying and see what they advise.
I have no idea if my advice is good, so please wight it carefully.
If these are texts rather than an open forum I would also get her to block the boys, but before she does that to say "My parents have taken screen shots of your messages and I am not blocking you."
If any of this behaviour is repeated in person, especially if it is in any way threatening, i would report it to the police. Again, no idea if this is the right advice but that is what I would do.