So when I was 19 I slept with a guy who was 45. It wasn't a relationship. It was two mates having a shag and he was brilliant. However. When I was 22 I got into a relationship with a man who was 40 at the time. We are still together 4 years later and having the best time. He was a mature student at my uni. We have similar interests, we go on amazing dates. Neither of us are weathy but I left uni last year (he's just finishing his MA) and started up my own business that I hope to go into partnership with him in. My family and friends were all really supportive. Which is good because it gave me confidence in the relationship, however, if we break up I know I can come to them for support not to be told that I was too young for him etc.
If your friend is healthy and happy, then let her be.
And sex with older men is incredible by the way.