Thank you, Ann. You may be right, though after trying to report this, I'm not certain.
The FB help center at
https://www.facebook.com/help/263149623790594 includes "How do I report a fake account that's pretending to be someone I know?"
First step, visit the fake account. I've deleted the message I got. The only account I find in my dead friend's name is the one she used when alive. I'm already "friends" with that account, and I don't see post-death activity other than people remembering her, so if someone has hacked the account, I'm not savvy enough to find evidence. Apparently the message came from a new account, which I can't find.
Might not matter:
"Please keep in mind that we can only act on reports from the person who's being impersonated."