What is wrong about a 15 year old going out with a 19 year old?

Im a 15 year old male and live in the u.k. Recently I have fallen in love with this girl in my class, I believe shes perfect for me and when I got her number I started to get to know her. Then she dropped a bombshell. When I was talking about previous girlfriends I casually asked her if she was single and she then said she had a 19 year old boyfriend and had been going out with him for 2 years. When I first heard about this I thought it was wrong but didnt say anything. Recently I asked her to prom and she said yes, since then I consider her as one of my bestfriends and when talking to her other friends I found out that most of them dont like the idea of a 4 year age gap in a relationship, they also told me they dont like her boyfriend and believe ill be a better match for her since we have a lot in common with each other. Then last week she came to me crying saying that she had just got out of a big arguement with her boyfriend and that this arguing has been going on for over a month. When hearing this I suggested ending the relationship and she answered that she loved him and didnt want to end it. This confused me becuase lately she had been telling me that she thought I was one of the only genuine guys around. Anyway im confused becuase I dont know weither the tell her about the wya I feel or just keep it to myself and see what happens with her boyfirend. In ther end I dont like her boyfriend and believe he is wrong for her, but I still hope shes happy even though it might not ever be with me. What should I do?