Q. (When making a Mi Sheverach for a sick person) If he is an assimilated cohen married to a non Jewish woman, do you still mention hacohen in the mi sheverach.
A. See prior answer, that in general you don't have to mention at all in a Mi Sheverach titles including cohen or levy. In this particular case, in principle, this individual is still a cohen, although, you may not call him to the Torah reading. His children from this wife will not be cohanim.
Horav Shlomo miller Shlit'a suggested that you should also pray for his "refuas hanefesh", spiritual healing and return to Judaism.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit"a