Primary residence capital gain tax after removing a co-owner

Hi,

 

Me and my mom co-own a house (50-50). She has been living there as her primary residence for more than 2 years, and I live somewhere else. I plan to transfer the title to her completely. My question is in order for her to take the full primary residence capital gain tax benefit (i.e. $250K), does she need to live there for another 2 years after the title transfer or she can sell the house immediately after the transfer and qualify for that benefit? Thanks!