Ask the doctor for a correct infusion for someone of her weight, or (because it's possible he doesn' know), contact the company that wrote the paperwork you have, which I assume is the company that developed it. It seems like it would still do some good even if it has been more than ten days.
The reason is that in the studies that have been done so far the data appears to show it only helps those who get it early after getting covid. After that the data didn't look so good. I'm not sure it's a known harm thing, but there are a lot of possible unknown harms in meds that haven't been used for a long time. That isn't just true for drugs only approved for emergency use, but also for drugs that go through the full approval. I'm assuming both of the monoclonal antibodies in existence will be fully approved because they work really well. But because it does only have emergency approval, docs are going to be more reluctant to use them otherwise because, frankly, nobody from the company is yet selling it to them for things the FDA didn't approve it be used for. They will, just wait. But we do have a very well known example of someone who did get one of them later in the disease's procession, and that was former President Trump, whose life was very likely saved by it. He was out of the window, I believe. Do see if you can find out if safety is a concern and if dosage varies for smaller people, but I'm guessing the company won't tell you anything and I don't know if your doctor is likely to have seen any data. You might be better contacting CDC or NIH, but again, if you do, by the time you find out it may be too late. I think the reality is, the perfect isn't available, and your doc is going to have to make a decision.