HELP!! my cats r fighting?

ok so i have 2 cats...one is myn...the other is my friends. were takin care of her while shes gone....myn i a boy and hers is a girl....we kept the girl inside and the boy outsside...we let the girl cat outside with the boy and were fightin like crazyy...he keeps scratching her and everything..wat should i do..my mom sais i have 2 put both cats outside cuz one still has fleas... wat should i do??