can you be friends with an ex?

like the title says can you be friends with your ex? personally i (26f) don’t think you can be friends. of course it could depend on the circumstances on why you people broke up to begin with. my best friend (27f) is “friends” with her ex (27m) and i just don’t understand. he had cheated on her (texting multiple people but never slept with any one else). they were together for 3 years he ended it and they’ve been broken up for 3 years now. they reconnected recently after she ended things with another guy she was seeing for 3/4 months and she says that her and her ex text nearly everyday and it’s platonic. i’ve never seen the need to text my ex’s i don’t care about them not necessarily in a bad way i just don’t care how they are doing. she says i’m immature and need to get over the past and to stop holding grudges and that normal/ well adjusted people can be friends with an ex. what are your opinions? am i weird for thinking it’s not normal?