He's too old for her!?

One of my good friends has ome out and told me she is seeing an ex-professor, who is just over 30 years older than her!! :-O She has only been in one actual relationship before, it's hard to not support her finally falling in love. Well' "in love." I think she is probably just in it for the thrill.. (he is married, they have to hide their relationship) What can I say to her to help her figure it out that he is JUST too old for her?? I don't want to be too mean.