Okay then, a non-troll response.
Chalotte you ae being very presumptuous in saying that you need to make your friend 'see sense'. Withthe background you describe it is likely that she is looking for some stability and she may have found it. You aven't said anything about the man in question: have you met him? Is he a good man? Relationships with large age gaps have been known to work. If youre really her frend you should support her and be there for her, that way you can check for any signs that the relationship is unhealthy, but you must also be open to the idea that it could be a good relationship.
This is in no way paedophilia. Paedophilia is sexual attraction to children before they reach puberty. At 17 (and indeed many girls at 14 or 15) your friend is physically an adult, therefore benig attracted to her is perfectly reasonable.