Lizard, you just need to write it all out and look at anything the school has about safeguarding and equality stuff.
Then you need to book to see the Head. Point out that a. the situation is more serious than a yr 7 girl can deal with. Explain nicely that they are at risk of failing their duty of care to the boy. (What are they doing asking a child to deal with this??)
b. Point out, again, that they are surely not in full possession of the facts because they/he/the Head has committed an act of gross sexism, and also come perilously close to enabling some kind of action that is really triggering alarms about sexual safeguarding ... he is notprotecting your daughter.
Seriously, it is soooo wrong - in a way it is only possible to get sooo wrong if you have zero idea of sexual politics. a. It is way to easy to tell girls to befriend troubled children - we have vaginas: we were born to make nice and care (for no money) b. Only a gender blind ... something ... would have jumped into a situation with a boy and a girl and not paused to reflect if there was a sexual issue ie. is coercing a girl to be "friends" in fact coercing a girl into a "girlfriend" situation.
Head sounds a bit out of touch.
By the way, I find it really, really odd that a HEad would get involved in something like this. I'd have thought Head of Year first, and for you to have been informed by the school/Head.
In all my dealings with my older child's school, issues of any seriousness have a sort of "grading" as to who deals with them, and how soon parents are involved. Head of Year incidents get a call home. An incident involving the Head would, frankly, be nuclear. I'd be expecting not just a call home, but an invitation into the school.
Really, if this has been going on, and there has been no direct line of communication with you ... this is just plain weird in this day and age. Frankly, there may be quite ... interesting ... issues with the school.
I think the whole thing is truly odd and disturbing.