When someone from Bosnia refers to themslef as Bosnian it usually means they are Muslim. Bosnian Serbs and Bosnian Croats generally refer to themselves as Serb or Croat.
There are many mixed marriages in Bosnia, had this person mentioned the town it would be easier to verify.
The only city of that size that fits the description is Mostar. The Siege of Mostar was very brutal, lasted about 9 months.