This is a very strange time for everyone, right now; that includes us as teachers, professionals, parents, non-parents, etc. We're all trying to figure out how to navigate this landscape, and often that means we have to be flexible and find out what works in practice.
I can only really speak from my experience in a low-SES, rural school situation. If I were in your shoes and there was a student or two whose grades were this far down the tubes, I'd do a couple of things. First thing I'd do internally is try to take stock of what your reasonable expectations can be of your students in a time like this. Based on your student population and factors like their SES and living situations at home, what can you reasonably expect from a distance learning situation? Keep in mind as well that it isn't just our classes they are going to try to keep up with; they've got their core classes as well. Next, compare the grades they have now with their performance from the rest of the year. Is this in line with how they've done before we went on quarantine, or is this a wild swing in the other direction from then? If this matches the work that they've done up to this point, I would strongly consider recommending to the counselor that the student repeat whatever level of band they had with you, if they choose to continue with the program. I'd also develop a more structured set of expectations for where you want this student to be at given major points in the year, and go over this with their parents/guardians so that everyone is on the same page. That way, you have contingencies in place for if they do or do not meet your expectations. If you do this, however, you absolutely must commit to what you have written down. I'd go over it with administration and even your campus counselor so everyone agrees on what's being done. If the student or their family disagrees to go along with the plan, that can be their consent to exit the program.
If this is outside the norm for the student in question, then I'd reexamine the potential environmental factors that could have contributed to their work output. For example, could they be in a situation where economic factors are requiring that they work wherever possible to help keep the family afloat? This is a long and varied question, and obviously only your knowledge of the student's situation can answer it.
Finally, the last thing I'd consider is this student's progress compared to the rest of their cohort group. If they're not outrageously behind by comparison, do you believe they could catch up to the rest of the kids if they were to continue onward with your normal internal program promotion? If you are afforded the resources necessary to get them up to speed early next year(providing, of course, that next year works out the way we're all hoping), do you think it can reasonably be done without doing the student, the rest of the class, or yourself a disservice? If so, that may be the ticket.
You may notice that I honestly didn't say much about the numerical grade in this post. Honestly, that's because I think that's the least important part of all this. A grade is a singular moment, whereas focusing on the student's development and progression has benefits that last much longer. This is my relative youth in the profession speaking here, but I think that expecting the kids to just go through this whole experience with anything resembling normal function is coming at it from a position of privilege. We have a better shot at doing so as adults, because we've got way more life experience to go off of. We also have more control over more aspects of our environment as we go about all this drama. My honest opinion about the grades is to give them the benefit of the doubt, and spend more time planning on how to best help the kids when we're all back together.
Apologies for the long post, but I really wanted to be as thorough as I can on this. I don't mean for any of this to seem like I'm being condescending at all.