I've had parents email me that are upset about a grade. I let them know of their options and encourage their student to come and talk to me. Sometimes it is a me problem and I try to fix it. Most of the time the students are not telling thier parents the whole story. I would encourage your student to advocate for themselves. If that is not working they may need to take it up a level. I've only ever heard a couple of times from administration about grades. Those times were when the student had not even reached out to me yet.