Most women I know who date younger (and much younger...) men are lacking somewhat in their ego... I know this is a very judgemntal statment but still...
I am sure there is the odd one out but doubt your friend is the one out. So - why telling her what you think? She will go ahead and date him anyway, resent you for your judgment and then when he leaves her she will feel very small.
Personally - I could not date a younger man and women at their 40s and 50s dates men at their 20s and 30s looks like a bit of a joke to me - but hey - each to their own... and who knows - they may live and learn (the hard way). Just be there for your friend when she needs you