Don't know what to do HELP!!!!!?

my niece boyfriend the father of her baby. is living with us. and so far he is treating her abd me with respect. but when i go off to work he hits her and beats her. how i know is because she has bursies all over her body. i dont know if i should kick him out or let him stay here in my house and take him to work with me so he dosnt hurt her. PLEASE HELP ME!!!!!!!!!!!!!!!!!!