My daughter

Our daughter of 18 has recently met a 28 year-old trainer at the gym. At first, I thought it was friendship but recently we discovered it is more. My husband and I told her that the relationship was unacceptable. He is too old. He is divorced with a 5 yr old boy. Our daughter is torn because she knows that we are upset about this. She says that she is not truly happy because we won't allow her to be due to the fact that we will not accept this. But ... she continues to see him. My husband refuses to get to know him. I have met him and he seems like a very nice person. I believe he really cares for her. Still, I can not deal with this. She has only dated one other boy in high school and I thought she would take her time as she started college this year. I see so many problems with this and have expressed them to her but again does not understand. Please help.