She was ditched by him like an old towel with no explanation whatsoever, and he’s waltzed back into her life saying sorry and telling her he still thinks about her all day and everything day and that she’s still in his heart.
She’s accepted his friend request. Although she didn’t accept it straight away, I think she accepted it 2-3 months after he sent it.
The only reason I can think of is that she still likes him.
Men always come back when they realize what they’ve lost, my ex from years ago did the same thing. But if he’s actually showing signs of remorse for what he did, then things should probably be okay, but keep a close watch. Actions speak louder than words, after all, so remember that too. It’s good that you are looking out for your sister.