What is going on with this?!?

It's a little odd I'm writing for a friend but bare with me. My best friend Steph has been dating this really terrific guy, he's great for her, and it seems their good for eachother. She's 19 and He's 27. (Age is somewhat an issue, he's at the age where he wants to settle down, shes at an age where she wants to go out and have fun) Problem is, he wants to marry her, they've been together for years so they've had time to discuss this over. She DOESN'T want to get married...at all, ever actually. She said she never wants to make that commitment (She's a stick to her guns type of gal, some would say she's just saying she doesnt want marriage because she's young and wants to have fun...but really, she HATES the thought of marriage) I know he plans to propose to her, and Steph said she will kindly decline his offer, but what's the point of dating if she doesnt want to get married? Shes NOT one of those girls that wants to date so she wont be lonely, she doesnt care at all about being lonely. Should they break up if they both want different things? Idk...