Not a legal expert, so consider this an educated guess.
Place of birth carries some importance when determining Croatian citizenship, but citizenship of parents is much more important and birth in Croatia does not in itself confer Croatian citizenship.
Until the early 1990s Yugoslavia consisted of six republics, of which Croatia was one. Every Yugoslav citizen was also a citizen of one of the republics. The republican citizenship was based on several factors and could change during a lifetime. For international purposes it was the citizenship of Yugoslavia that counted.
While a citizenship of a Yugoslav republic did not always translate into a citizenship of the independent successor state, as each of the new countries has it own nationality law, if your girlfriend did posses Yugoslav-Croatian citizenship (rather than e.g. Yugoslav-Bosnian) she would have a strong claim to citizenship of the independent Republic of Croatia.
Also helpful in claiming Croatian citizenship would be proof of belonging to the Croatian people. If she can show that she identifies as Croatian or that at least one of her parents were culturally or ethnically Croatian her chances could improve considerably.
Edit as response to comment:
Under Croatian nationality law, a person born in Croatia with one parent being a Croatian citizen at the time or birth or born anywhere with both parents being Croatian citizens is automatically a Croatian citizen.
If your girlfriend can show that she was registered as a Yugoslav-Croatian citizen or that she should have been by birth in Croatia to at least one Yugoslav-Croatian citizen, this may be enough to be considered a Croatian citizen.
According to the Ministry of Foreign and European Affairs:
The applicant should state the reasons for applying for citizenship, their relation towards the legal order, customs and acceptance of Croatian culture; depending on the legal basis for submitting the application, where his ancestors were domiciled before leaving, when and why they left the Republic of Croatia, when and with what documents they settled abroad, the ways in which they showed they belonged to the Croatian people; participation in sports, cultural and other societies promoting Croatia’s interests abroad (Article 11 and 16).
Croatia obviously wants to confer citizenship on ethnic Croatians. Which arguments to use would be highly dependent on individual circumstances.