It's not just about his age, there are lots of worrying things about this, so some relationship pointers not related to his age might make her think:
-a general conversation about red flags, moving too fast, asserting control;
-the need for her to keep up her own life with her friends and family alongside the relationship;
-it's important to recognise that the material things he provides are not a reason to have or stay in an otherwise unsatisfactory relationship, she shouldn't rely on them or feel beholden to him for them;
-in any relationship she deerves to have her needs met too including have autonomy and a chance to grow as a person, decide who she is and what she wants, have good sex, make key decisions;
-she's got to think of herself and take responsibility re career progression, having savings to fall back on, training, learning to drive etc irrespective of the relationship she's in.
A friend of mine spent all her 20s and part of her 30s being completely boyfriend-dependent (always moved in with whoever it was, had no savings despite a good career, no pension, no flat). When her worried father asked her what she'd do if the latest one left her, she said 'Move in with Eldritch!' She thought this was hilarious, I didn't. She married that boyfriend, if he ever leaves her I don't know if she'd survive. It's an awfully risky pattern to get into.