Can I visit the same schengen country twice with a single entry visa?

This is not about me but my dad. I've come to Romania to study and my dad came along with me just to help me settle in my apartment. I have an older brother who's in Hungary and my dad wants to visit him for a short while. However his return ticket to home has already been booked through Bucharest. My question is can he re-enter Romania after going to Hungary with his single entry visa? I'm sorry if this sounds like a stupid question I just don't know.