Ooops!
I am an ex teacher, pastoral care too! So please bear with me as I am not condoning the error or lack of resposne to the initial issue!
The HoPC will only have the HT/HoD to talk to relatively freely. They will be her release valve, iyswim.
That kind of comment is often made after a fairly heated head to head oith a student, followed up by an email from a parent. The sort of thing a 'Justice Warrior' student with a be in his bonnet will elicit if/when he goes off on one. Which they do, it is the nature of the beast.
You say you have told him that if he is going to make the teacher look stupid he should do it in private not in front of the class. Well, that shows he is 'one of those' students who don't yet know when to stop talking and start considering why they aren't getting the repsonse they want and that you are supporting him in the belief that he has a point, but should make it at a different time.
So maybe, and remember I don't know the seriousness of the reason for original email, the HoPC is right in her thoughts but horribly wrong in her email skills!
Either way, she will be extremely embarrassed and will get a verbal reminder of professionalism... BUT what you say in your meeting will depend on what you want as an outcome. DO you want her pnished or do you want the original issue fixed?
Trust me, there is nothing much more you can do/have done to her than she is already puting hersewlf through, so assuming the latter I would ask her why she was less than sympathetic with the initial issue, find out why it hasn't been resolved to your satisfaction.
Also remember you are furious that they are bad mouting you and your sone... but here you are opening them up to far more public, if anonymou, badmouthing! Everyone does it. Often it is entirely justified, sometimes it is just venting! Mostly it is just 2 people with entirely different perspectives on a happening.
Good luck getting it all sorted.