Call them out. In a case like this - it was clearly done maliciously - the community needs to know and if the perpetrator is not unmasked they will do it, or something similar again.

Nasty, sneaky people like this take advantage of the goodwill of the community and expect not to be unmasked - if they thought there was a risk of being called out in public they wouldn't do it.

A conspiracy of silence allows people to get away with it over and over again. We must accept that there are going to be bad apples in the community and be able to deal with them.