I need advice to give to my lil sis

So my little sister has a bf and he recently has been acting different and he's very sneaky because he does not express the way he feels. He talks to other girls expecting my little sister to be okay with that like he gives them hugs and he lend this girl that he gave a hug to his hat and that got my little sister mad. He usually hangs out with her during school but this time he told her to hangout with her friends and he didn't tak to her after that. What advice should give her and what should she do?