Can a boyfriend and girlfriend file jointly if they have only been living together for 8 months?

I'm trying to get to the bottom of a mixed up tax situation....Can a boyfriend and girlfriend file jointly if they have only been living together for 8 months? Is it more advantageous to file jointly? My daughter's boyfriend claimed her as his dependent and he as head of household, but she was living with me for 10 of 12 months in 2010. Confused! Thanks for everyone who helps!!