Define “failing”. If they can get back to a D this quarter and then earn a B in 4th quarter, the semester grade will be a C. That is not the end of the world for an AP class. After AP tests classes are usually do a project those last 4 weeks, which is a good opportunity for boosting the grade. Have DC stick with it, finish the course, and earn the credit. They might decide it’s not worth worrying about the grade and not bother retaking it. See what the test score is, and if it’s not at least a 4, then self study for review and retake the test.

Can kid retake it next year, OP?

It's already too late. It'll show up on their transcript.

See details here: https://ww2.montgomeryschoolsmd.org/departments/policy/pdf/ikcra.pdf

Transcripts shall include the following:
a) Any MCPS course completed or dropped after the end of the 25th
instructional day of the semester, showing the date of withdrawal
from the course and the grade at the time of the withdrawal.
b) Only the highest grade and credit of a course completed and retaken.

We had a family issue earlier this year, and my kid is now failing a class so badly I'm not sure he can really recover. When is the last date to withdraw from a class so that the grade won't show? (And then I assume it will show a blank on the transcript or a dropped class or something?)
Alternatively, if my kid takes the same class next year, will that grade replace this one on the transcript?
It is an AP elective, if that matters.
I assume it won't help the GPA any if the kid gets a 4 or 5 on the AP?