My husband is 15 years older than me. When we got together I was 20 and he was 35.(We have been together very happily 22 years). People said all kinds of things about him being a dirty old man,etc. Everybody and their brother tried to break us up.( A few almost succeeded.) When I met him I was soooo tired of boys. I wanted to date a man. A man that new how to treat a lady. A man that knew what real sex was( I was so done with the "rapid rabbit" sessions! )And , yeah, a man with a few bucks so maybe we could have a real date instead of taco bell or mcdonalds.
We stayed together in spite of it all, and I've been blessed with a great man.
Bottom line : She is a big girl and should be able to live her life as she sees fit. Support her. Don't judge. And dont get in the middle. She will resent you for it. Its her life.