What did the FB message say? I think (without knowing the content) that no response at all is best.
My H also has an XAP that is nutty. Her BH knows and we chose not to send her a NC letter due to their collective craziness. We've had contact from both of them and responded only with crickets. Actually, once when the BH left a threatening message on my H's phone we went to he police, but we never respond to them in any way. It seems to have worked, no contact in months.
I think your H blocking her was response enough.