Information from Ministry of Foreign Affairs. http://www.mfa.gov.rs/en/consular-affairs/citizenship
Your Mum may apply for citizenship by origin. You may have to apply for citizenship by admission – a lengthy procedure. Serbia is also one of those countries that requires an applicant to release citizenships of other countries before applying for citizenship of Serbia by admission. Your Mum may have two citizenships. That’s how I understood.
I would consult a lawyer who knows about immigration and citizenship laws. Clerks and secretaries in offices often don’t know the subject well enough.