Last I heard, Serbia is not accepting non-resident Russians.
They mean legally, i.e. has actual residency. Doing visa runs on a temp stay status does not count as "living" in a place legally.
This is in line with my previous statement. They don't seem to be taking non-residents. Having legal residency in Serbia would indicate she can interview there.
K-1's are treated the same as immigrant visas in terms of the interview.