Advice on cheating

My sister is 20 her bf is 25 they have been together a year . At noon today he told her his grandma on his dads side passed so she was sad too and asks me to take her to get some stuff to cheer him up so we running around all day getting him stuff to cheer him up like clothes at the mall, his favorite food at his fav restaurant, gift cards to his fav place(target), and she gets dolled up to spend the night with him. I drop her off over there and not even 2 hours later she calls me crying asking me to pick her up from some random street in like why what are you doing there ? She doesn’t say so I go pick her up and she tells me she gave him the gift and he was happy but he told her he didn’t deserve it and to take it all back but he didn’t wanna say why . And he sat her down and admit he messed up . An ex from last year txt him said she was in the area at 7am that morning and she wanted to chill and they caught up and she kept playing with him and “one thing lead to another” and they had sex . my sister threw the food at him and she left and called me .

She wants advice on what she should Do. I say leave but she’s saying what guy openly admits cheating and is thinking about working it out ...

Update: his grandma actually passed he had sex with the girl 7am. But didn’t talk to my sister till noon