When to ask for LORs

Hi,

I have a situation where one of my letter writers was unfortunately diagnosed with cancer. I won't be ready to apply to law school until at least next cycle, but that depends on when I finish my PhD. So I wonder if I can have this professor write the letter and submit it to the LSAC system right now to be safe? I will have this discussion with this professor soon and just want to know what my options are at the moment. Thanks!